The Greater Internet Fuckwad Theory is relevant here:
https://www.penny-arcade.com/comic/2004/03/19
Except it isn't actually anonymity (or pseudonymity) per-se, as the phenomenon has long existed in many other contexts as well:
https://youtu.be/kFHT1lw3vSI
You can probably generalize to something like "technologically mediated assholism", but there is more than one thing happening here, such as:
+ Filter bubbles/homogeneous in-group opinions lead people to becoming more extreme in their views.
+ Observing some rando (it doesn't have to be an authority figure, although that helps) "say the quiet part out loud" is validating, to say the least.
+ Lack of f2f contact leads to thinking of individuals as impersonal abstractions, and in particular, as the "other".
+ The pathetic fallacy applies to groups just as it does to inanimate objects.
+ Text communication often causes people to misread tone.
+ The affective fallacy (the assumption that the effect of an action was the intent behind it) becomes particularly powerful because of the former two points.
+ The affective fallacy seems to operate in reverse too: Someone informing you of an outcome seems like they are making a claim about your intent.
+ No-one is a villain in their own story. On the internet, everything is a story, and if the story is about someone like you, obviously they can't be a villain either.
+ Exposure to information that conflicts with (even weakly held) beliefs tied to a person's identity often triggers defensive rejection of the new information and strengthens those beliefs.
+ Group dynamics are in play for public interactions, so there is always a performative aspect even for ostensibly 1-to-1 communications. This often doesn't get turned off for private electronic channels such as email.
And so on. A lot of these mental quirks are like ratchets that only ever tighten. Although each in isolation is often self limiting, the limiter can be bypassed by some other quirk, after which the ratchet can tighten some more.