I don’t know what exactly causes it, but I can definitely believe that it occurs.
This is a completely anecdotal story, but when I was playing football in high school, I remember one particular part of a game, where I got mightily irritated with the way things were going, and something flipped. I remember the rest of the game as a sort of out-of-body experience- it was definitely a different experience than any of the other games.
Combine that, with the fact that once this happened, I was suddenly much better than I’d been up to that point (or after, unfortunately). I got something like 2 sacks and several tackles during about a quarter and a half as a defensive tackle. I remember not really having to expend much effort to get around their guard’s blocking, and it seeming like I was always in their backfield. I don’t really remember any pain either, which was odd, because as a down lineman, there was always something that hurt a little bit.
Eventually, it wore off or something- I think our offense had an extended drive, and I got a chance to rest for a while.
Still, it was a completely weird feeling, coupled with superior performance and no real pain. I have to figure that the “berserker” rage could be something similar to what happened to me.