[Loren]

I don't think that's causing anything. It's OK to turn your head side to side, not OK to move it back and forth, up and down.
The head stays centered between the feet, preferably stationary.

Your two eyes focused on the aiming point of your hands (normally the aft inner quadrant of the ball) will tell you if your head moves, if you pay attention to it for awhile.
Notice the difference in feel when it doesn't move. See if you can describe that feel to yourself. Reproduce the feel. If it doesn't work, re-describe the feel until it does. Eventually it will fade into the background.

Shadow-boxing, er, swinging. Another way to check it.

What do you think of the amount of knee bend, compared to, say, a full-body forward press before start up, mimicking impact conditions?
Head height can be checked there also.