How do I fix?
Fix your weights. You have too much of some other bone (probably neck) on the head verts.
Thank you. So, you have to make sure that the vertices don’t overlap or it’ll cause weird stuff?
You have to make sure that you have good weights. The right way to do that is to look at your weights and edit them manually. There are techniques whereby you’ll get better results from autoweights than you would otherwise-- like don’t overlap mesh, sure, among others-- but you should not be thinking of autoweighting as the end of weighting, but as the beginning.
Ok. Danke.