Your jaw muscle tire out, and recruit other muscle
groups to help carry some of the work. Its that same thing with any muscle
group, as you notice as you are working out and get to the end of a set, your form is off, as your body subconsciously recruits other muscles when in need. Now the problem is that after a while, these muscle start to stay in spasm instead of constantly relaxing. This changes your alignment and causes some muscles to be stretched and others to be shortened.