I don't know that there was any specific catalyst, per se. But one of the more insightful takes I've seen on why discourse seems to be more contentious now is that a wide range of topics are now regarded as moral issues, and it's much more difficult for people to compromise on positions (or countenance alternative positions) when that's seen as abrogating their morals.
I think that there are a few, different reasons.
The first is the nature of communication on the internet. Generally, the internet (as a medium for communication) abhors nuance. In addition, the post/reply/reply/reply nature of forum communications tends to exacerbate and reward differences of opinion, as opposed to finding common ground- instead of
thesis - antithesis - synthesis, you get,
thesis - antithesis - more antithesis - even more antithesis - yo mama. Finally, it would appear that the internet prefers wise-cracking to wit; and as we all know, wit has truth to it, while wise-cracking is simply calisthenics with words.
The second is what you correctly note- the increasing use of morality for all arguments. When you certain arguments or viewpoints are believed to be not just incorrect, but to be
immoral, then it becomes easy to attack the individuals making those arguments as
immoral. If, for example, you think that people who enjoy pineapple pizza are not just terrible bard-lovers, but also immoral bigots, then you will likely find it easier to behave cruelly to them (and those people, in turn, will behave cruelly to you).
Third, I think that there are an increasing number of "real-world" divisions that are unfortunately getting reflected in all aspects of life. This board, with strict enforcement of rules, avoids that (good!) but that background antagonism seems to seep into a lot of things. But for various reasons ... people are just angry.
Fourth, and finally, impersonal communication through text tends to make it easier for things to escalate. You don't hear the tone of someone's voice. You don't see a twinkle in the eye. You don't see the very real reactions of people that you might hurt in your quest to pwn them.
I am sure that there are other factors people will point out, but those are some of the factors that occur to me.