It sounds like the Catharsis Hypothesis, which states that the release of aggression in small ways prevents large expressions of aggression. This has been rather thoroughly discredited. Small displays of aggression (e.g. ranting online) train an individual that aggression is an appropriate response, and leads to larger displays of aggression (e.g. physical attacks).
I provide an alternative hypothesis - most people are basically lazy. I don't mean that in a very negative sense, but that our brains are wired to take the path of lesser resistance whenever possible. I forget where I read this first, but I have seen it corroborated by academia. It is INFINITELY easier to type out a hateful message and be agreed with by your peers, than to do something violent and risk real repercussions.
That has nothing to do with laziness though, and everything with risk perception.
Still, if micro-aggressions aren't checked, it is easy to assume they are accepted. That will lead to an escalation of aggression like the GP described.
Source: http://www-personal.umich.edu/~bbushman/PSPB02.pdf http://blog.uwgb.edu/alltherage/four-questions-on-the-cathar...