What would cause jaw popping after having cavities filled?

Bite off/jaw fatigue: Unless you had popping in jaw before it may be do to the filling being slightly higher than your bite prior to the treatment and that would cause your jaw to be misaligned. Also if it was a rather long treatment it may be do your jaw muscles being stretched and causing the jaw to pop on opening. If it is from fatigue it will stop over time, if the bite have it adjusted by your dentist.

Open too long: If you had your mouth open for an extended period while having the fillings done, this may cause a dislocation of the disc of the TMJ off the top of the joint. The popping you hear is this disc sliding back into place on the top of the joint. This should be a temporary condition as healing occurs. If the disc has become significantly displaced, you may need splint therapy to recapture the disc.

Disk: TMJ (jaw joint) disk out of alignment. Due to prolonged or too wide open dental procedure. Should settle. If does not settle down, or you have jaw pain, ask dentist for referral to TMJ expert. Any dentist can be a TMJ expert with the proper training and experience. Most commonly, oral surgeons, prosthodontists, and orofacial pain specialists. Ask MD, dentist or your dental society for referral.