In all the years I have treated teeth on horses, I have seen a lot of injuries in horses mouths.
These injuries have come from lack of dental care such as sharp points on teeth, tooth tips, wolf teeth or wrong tooth position. Others may have come from the wrong type of bit or wrong width of bit.
Types of headgear and design as well as tightening of nasal bands can also have an impact on wounds in the horse's mouth during riding.