Well - its partly from the grinding which is a symptoms of the tmj as well. Grinding is a involuntary form of exercise for the jaw and muscles - over time they get stressed out since they do not go into a "relaxed" position or even if they do its not long enough for them to recouperate. Grinding also over time rounds the edges and flattens the surfaces of teeth - sometimes significant enough to lower the vertical height of teeth (which in turn affects the positon of the tmj joint) and this in turn can impact a person's equilibrium. So between the muscles being overworked and tired which can lead to inflamation as well (this can affect the eustachion tubes) and the changes in bite that occur from chronic grinding chances are pretty good the dizziness is stemming from the "tmj".