I've seen campaigns fall apart due to:
1) Lack of Planning - No set time and place for the game, and instead of saying "Okay, meet back here, next Friday at 7" at the end of the game, someone said something like "We'll get together, later, and plan our next game". I objected that it would never happen (as it hadn't in the past), but was over-ridden... and it didn't! (We lost more good campaigns, that way!)
2) Playin' with People Who Aint Yer Friends - I played in a game with someone I thought was a friend, at one point, but he apparently didn't agree. I greeted him, when he joined, but he didn't respond. I thought that was odd, but shrugged it off. When his PC showed up, I welcomed him, and again received no reply...
Later, when I had to leave the game for a while (moving), I used the in-game excuse of my PC not wanting to pay the gate toll to enter a Human city... This prompted another PC to attack mine, then the "friend" to begin bad-mouthing my PC for the rest of the game (even after I had left it). The death of the campaign followed not to long thereafter.
Now there may be more to it than that, as (during the campaign) we discovered that some of the PCs in our group were spies for various organizations... but that's another point!
3) Player Distrust - Back in college, I knew three guys who were roommates who you NEVER wanted to invite to your games... They had back-stabbed each other in so many games, that they could bever trust each other!
In a campaign that I ran for almost two years, I succeeded all to well in attempting to induce an air of paranoia... I did such a good job, in fact, that when one PC lied to the group about where he'd been and what he'd been doing, and another motioned to his buddy (holding a finger to his lips, indicating silence, and shaking his head no, that what the speaker was saying was untrue), the other PCs assumed that the two of them were in cahoots, and somehow channelling them away from "the right answers" in solving the mysteries at hand.
The group then fractured into tiny little pieces. Two PCs had gone off, before, but come back after running out of magic and getting slightly hurt. After they'd gotten together, again, this one PC had taken off by himself, done some searching, and distrubed a bull, whose bellowing summoned the others. He then lied, claiming that he had been teleported.
When confronted, the other PCs sided with him against the two who they figured were in cahoots, said they were running things, now, blah, blah, blah! One of the two PCs sided against (the one who confronted the liar) then left the group. The other went after him, but never found him. A third PC went running after the second, then back and forth between him and the group. Another also went off by himself, remaining alone (and actually finding some useful information), until he was relieved of all his weapons. A fifth PC then lied about hearing a noise, and led the final PC off to "investigate".
At one point, every PC in the game was off running around by themself! Major GM headache, for me!
After some player drops, things calmed down, but the group never did get much of anywhere, after that... In the end, the BBEGs won, and the only survivors were the two PCs suspected of being in cahoots, who abandoned the quest and bugged out!
4) RL - Yes, I've seen this destroy otherwise-successful games, as well. Less so than other problems, though.
5) Bad GMs/Player-GM Disagreements on Style - Differences in style, especially when coupled with the below, often kills a campaign.
6) Too Few Players - You CAN run a game with just two people, but it's not that much fun! Even with three or four, there are always times when someone can't make it. With three people, and one missing for a while, it's doubtful that the GM and one player will go ahead!
7) Too Many Players - I have sat in a game with one GM and more players than I could count... In the range of 20, I believe... It was ridiculous. The GM could only deal with a few at a time, and everyone else sat around bored, waiting their turn. Fortunately, this was a one-shot, and the GM said "Never again!"
I'm sure there are other campaign-destroyers, too...
1) Lack of Planning - No set time and place for the game, and instead of saying "Okay, meet back here, next Friday at 7" at the end of the game, someone said something like "We'll get together, later, and plan our next game". I objected that it would never happen (as it hadn't in the past), but was over-ridden... and it didn't! (We lost more good campaigns, that way!)
2) Playin' with People Who Aint Yer Friends - I played in a game with someone I thought was a friend, at one point, but he apparently didn't agree. I greeted him, when he joined, but he didn't respond. I thought that was odd, but shrugged it off. When his PC showed up, I welcomed him, and again received no reply...
Later, when I had to leave the game for a while (moving), I used the in-game excuse of my PC not wanting to pay the gate toll to enter a Human city... This prompted another PC to attack mine, then the "friend" to begin bad-mouthing my PC for the rest of the game (even after I had left it). The death of the campaign followed not to long thereafter.
Now there may be more to it than that, as (during the campaign) we discovered that some of the PCs in our group were spies for various organizations... but that's another point!
3) Player Distrust - Back in college, I knew three guys who were roommates who you NEVER wanted to invite to your games... They had back-stabbed each other in so many games, that they could bever trust each other!
In a campaign that I ran for almost two years, I succeeded all to well in attempting to induce an air of paranoia... I did such a good job, in fact, that when one PC lied to the group about where he'd been and what he'd been doing, and another motioned to his buddy (holding a finger to his lips, indicating silence, and shaking his head no, that what the speaker was saying was untrue), the other PCs assumed that the two of them were in cahoots, and somehow channelling them away from "the right answers" in solving the mysteries at hand.
The group then fractured into tiny little pieces. Two PCs had gone off, before, but come back after running out of magic and getting slightly hurt. After they'd gotten together, again, this one PC had taken off by himself, done some searching, and distrubed a bull, whose bellowing summoned the others. He then lied, claiming that he had been teleported.
When confronted, the other PCs sided with him against the two who they figured were in cahoots, said they were running things, now, blah, blah, blah! One of the two PCs sided against (the one who confronted the liar) then left the group. The other went after him, but never found him. A third PC went running after the second, then back and forth between him and the group. Another also went off by himself, remaining alone (and actually finding some useful information), until he was relieved of all his weapons. A fifth PC then lied about hearing a noise, and led the final PC off to "investigate".
At one point, every PC in the game was off running around by themself! Major GM headache, for me!

After some player drops, things calmed down, but the group never did get much of anywhere, after that... In the end, the BBEGs won, and the only survivors were the two PCs suspected of being in cahoots, who abandoned the quest and bugged out!

4) RL - Yes, I've seen this destroy otherwise-successful games, as well. Less so than other problems, though.
5) Bad GMs/Player-GM Disagreements on Style - Differences in style, especially when coupled with the below, often kills a campaign.
6) Too Few Players - You CAN run a game with just two people, but it's not that much fun! Even with three or four, there are always times when someone can't make it. With three people, and one missing for a while, it's doubtful that the GM and one player will go ahead!
7) Too Many Players - I have sat in a game with one GM and more players than I could count... In the range of 20, I believe... It was ridiculous. The GM could only deal with a few at a time, and everyone else sat around bored, waiting their turn. Fortunately, this was a one-shot, and the GM said "Never again!"
I'm sure there are other campaign-destroyers, too...