Occurs because of two main reasons:
1: An intense Team Deathmatch is going down causing your pits to become wet and soggy.
2: That camping noob has pissed you off to the point your rage has heated your body and caused you to perspire.
After 3 hours of "intense battle"
Gamer 1: Ah man, that was close. Can't believe we won it.
Gamer 2: Yeah. All right I think I'm gonna get off Live for a while. *stands up and looks at his arm pits*
Oh shit dude, I got some real Gamer's sweat!
Gamer 1: Yeah, me too.