In a way, I do think so, yes. But not necessarily because of selfishness or ego (which are the two I find most annoying about traditional self centredness).
To give an exaggerated example, someone whom crashes his/her into a fence and breaks his/her leg, is naturally going to be more concerned with him/herself rather then the person whom has to fix that fence. In this scenario the person in question is being self centred, but for self maintenance reasons.
I think that in a less severe way, that's what happens here too. People feel trapped and cornered by fear, triggering their self defence instinct. What is fear/anxiety if not that? And perhaps after a while of being here, it unconsciously turns into a habit to focus on one's self.
And of course, many people with social anxiety live life quite isolated. On a general day they might not need to invest that much time into others, making general social behaviour an alien thing to them. Frustration in combination with the power to finally express themselves on a open forum (literally) might not be the best motivation to inspire proper social behaviour.
It begs the question, how much venting is harmful to your progress? And is it perhaps beneficial to sometimes, forcefully, invest some time into other members.