Facial pain (dental , sinus), back pain, lameness, rough handling through the reins, size or type of bit or where pressure and amount of pressure (no release or slow release from bitless) occurs... Lack of training and not broke as well as you think. Could be very green and unsure.
All sorts of reasons. Most typical I would say is the rider and level of training in combination.
Some horse people change their horse, they change their tack and discipline, they change their instructor; they never change themselves.