You have to realize that not all of the fans of other teams that are here are what you would consider trolls. There are a few oiler,shark,wing,wild, and even habs fans on here who actually contribute positively to the board, and are accepted members here. I am assuming you don't mean these people, just kind of clarifying that.
As for the others, generally they sign up to gloat when their team happens to be beating the flames in a particular game, generally to be banned, run off, or just flat out disappear once either the game is over or the Flames come back, or even when their team gets a beatdown next time the two teams play. They seldom last long enough to read any threads other than game day threads, or threads pertaining to their own team/players.
To your question about if it's a common thing or not, I'm pretty sure this happens on all sports team's forums. I don't know for sure, since I don't go to any others, but I would expect there would be fans of all teams in all sports who feel the need to trash talk their rival's fans.
Oh and for the record, all canuck and leaf fans are trolls, even on their own forums
