Oh hey, that was me. Yeah, it's random I'm sure, but I do just seem to have an abnormal tendancy to get vamp. I actually had it turned off for a while because it happened almost every other round - my longest vampire streak was four(!) games in a row.
I heard somewhere that the system was weighted a little bit to make people who hadn't been antags in a while more likely to get it, but I don't know how true that was, and it doesn't seem to hold water since it really does tend to be the same bunch of people antagging every other round.
The fact these people do play a lot though probably adds to it - 'course someone who plays like 5 rounds a day is going to get antag more than someone who plays once. It's just more chances for it to happen.