The thing about Twitter is that everyone is connected to the same party line.
Imagine you are at a party of 100 people, all having their own small conversations in of two to five people. Then imagine someone throws a switch and every single thing said is heard by everyone in the room, but only fleetingly and with no time to imagine the context.
And sometimes Twitter's culture makes it really hard to find context. If someone responds directly to a tweet, you can see that in the conversation, but a manual RT destroys that chain.
The real solution is that we have to recognize that our lizard brains love beating the shit out of people. We've put on a layer of civilization that stops us from doing that in person, but it's just as bad to do it online. Once we recognize that we can be monsters, we can figure out how to stop doing it, or at least do it less.