What happens in our brains during extremely intense, stressful, physically-exhausting situations, that causes us to forget said situations entirely?

I see it in MMA (mixed martial arts) all the time: post-fight, a fighter will have absolutely no recollection whatsoever of his fight, without even having been knocked unconscious or 'rocked' at any point during.

And from personal experience, I've been a vocalist for a few hardcore bands. Our shows were always very intense, and quite often, after a performance, I'd have no recollection of it, almost as if I'd blacked out. Is there a scientific explanation for this?