Football is an intense and passionate game--if you play it right. If you are into the game and focused on what you are doing (like trying to win) it can be pretty easy to set someone else off or get set off yourself.
Apparently at some point in the first half that happened between Bears kicker Robbie Gould and kick returner/wide receiver Devin Hester.
If any two players should be cool at all times I'd think it would be the kicker and the kick returner. Gould doesn't kick to Hester, but there are not too many guys that are real specialist like they are.
Plus the thought of a kicker fighting is just down right silly--although from what I read on Twitter most folks think Gould could take Hester.