They all showed up when I moved my king to the h-file last night, thinking that it would somehow win me a pawn with only kings and pawns left on the board.
I've basically concluded that people watching your game is the IRL equivalent of the computer giving you a -72.0. (If you're playing white, that is.)
It may be a herd mentality of trying to defend the group from outsiders, thus they show up to see the established member of the group crush the new guy (or gal) and thus hope that their group integrity is preserved and that the outsider will be driven away. That's probably the general theorem.
If your club is connected with the university, freshman vs. senior class dynamics also can play a role. And if you going off to FIDE tournaments and getting instruction and all of that is known, and if your enjoy winning and telling people about all of your chess achievements IRL as much as you do here, people like to see the braggart guy (or even the guy with all the advantages) get his comeuppance or make a stupid mistake. "He thinks he's all that? Ha." If even I, with my philosophical sense of the long run, can feel this sense of wanting to beat you, a bunch of unbelievers will basically get thrown around by it like leaves on the wind. (Not to mention that them showing up puts pressure on you because you're now on a stage and everyone is watching which can cause you to mess up even more.) They all want a piece of the satisfaction.
Finally, there is also the desire to assert superiority by giving advice to the losing player after the game, and that's how I found my instructor. There might be other things going on there, including plain old suspense, but that sums up a lot of it, I think.