Two things:
Using the brakes too much.
Or
A frozen or seized piston.
Its more likely that you have a frozen or seized piston. This will cause the piston to not retract and make the pads rub on the rotor all the time. This could wear out the pads or even make the brake catch on fire!
The fix will require either replacing the caliper with a new/rebuilt one or rebuilding the old one yourself. It is always best to do brake work in pairs, do both brakes on that axel. Its safer that way!
Using the brakes too much.
Or
A frozen or seized piston.
Its more likely that you have a frozen or seized piston. This will cause the piston to not retract and make the pads rub on the rotor all the time. This could wear out the pads or even make the brake catch on fire!
The fix will require either replacing the caliper with a new/rebuilt one or rebuilding the old one yourself. It is always best to do brake work in pairs, do both brakes on that axel. Its safer that way!