I finally figured out what makes the melee in this game suddenly wrench the camera in a random direction (usually towards the sky) - if there is an enemy close enough, the melee "locks on" to them and you automatically turn to hit them. The problem is that the lock-on is ANNOYING as hell and extremely disorienting, often leaving you with no clue where you're facing or where your enemy went.
At first I thought it was supposed to be disorienting (as a balancing factor), but then I realized that punching air doesn't make my camera go wild - only when I hit an enemy. So it is purely the side effect of a terrible lock-on system that isn't even supposed to be there.
It was NOT there in Crysis 2 so I can only think of one reason: The auto lock-on was aimed at consoles, because in close quarters things get VERY fast-paced. On consoles people simply can't turn around fast enough to melee an enemy who's bunny-hopping all over them. It's just a fact, can't be helped and I think the lock-on is a great solution!
But for some absurd reason PC users got stuck with that feature as well. Keep it simple, revert it to how it was in Crysis 2! With a mouse we can turn to face any direction instantly, the help is not needed, it only serves to annoy. Or at least let us disable it. This was clearly a feature aimed at consoles.