I think it all comes back to not just the killing of D&D sacred cows in 4e, but doing so with literal glee and pride. It was both the content of the changes, and the way in which they were presented that caused an even deeper fracture.
As to the 3E/4E schism, I'm curious as to opinions: did the change in flavor or mechanics cause the major split?
The problem is twofold. Biggest is the problem of rivalry.
The advertising BEFORE 4th edition came out and the "blurbs" on what was in and out and what was "wrongbadfun" about 3rd in the marketing probably caused the most damage. (cont...)
I thought the break up happened when 2nd edition came out.
And I'm only partly joking.
(...cont) Each new edition caused some split, 2nd had more players than 1st because after a while support for 1st ended and people could only get 2nd. While they were mostly interchangeable, getting the rulebooks for 1st became impossible for a while. The internet wasn't that big, but had it been there would be the same arguements today about the two of some points, but overall many people would agree on many things in both additions and disagree with other things in both editions. So they could cohabitate.
Many of the 3rd edition changed caused the system to not be interchangeable, dropped the word "Advanced" from the game, and pretty much showed signs of saying "AD&D sucked so stop playing it". 3rd gained a mountain of players unlike before, and the internet gaining ground there were more places to discuss things. Since 3rd was presented in a, less nice way than 4th; it caused the first major split with the larger number of players ocming in and the internet.
While AD&D may have had separate communities, they were still not fighting as loudly all over the place that the ripples could be seen. The fighting was there on FIDOnet, USENET, Prodigy, AOL, and CompuServe about some things, but not the attempt to say your edition sucks so stop playing it cause of this and that.
4th just came out and said 3rd edition sucked so stop playing it, and that, again since there were more player for it than previous editions, and because the internet was nearly in every home, pretty much did the final damage.
Fluff changes to some settings to retcon them caused people to get upset, mechanics weren't in a fashion to anything like they had been before, and MANY saw the 4th edition to be a game with D&D elements, but wasn't D&D, while it tried to say everything before it was not D&D. Then you also had forums around for years that people had made friendships and homes at and D&D was stabilizing around a game/edition finally when BAM here comes 4th to shake things up, and that it did.
When there is that much force and the load causes that much stress, something is going to break. Well the dam broke, and not to demean the Australian situation right now, but we are all in a flood caused from that damage waiting for it to dry up a bit when the levels go down.