When using headtracking, if you vault over stuff, fall down in elevation, or otherwise do certain actions; the camera goes all fucky-wucky and makes you look down, up after a few seconds, then down again. Even if you wrestle for control with the mouse (and manage to win) it still misbehaves.
[Using Opentrack to headtrack]

A temporary solution to that is tapping your freelook key for a second (Default: L-Alt) and it fixes the issue.