Help! What has happened?

I used to have a Nissan 200SX with a similar rear caliper design. You had to rotate the piston to retract it for new pad installation. After a few years, I found I could not rotate the piston all the way in, so had to grind off some of the pad material to fit them in.
The cause turned out to be corrosion of the outer surface of the piston in the area where it protruded from the cylinder. Surface pitting prevented it from being pished back into the bore. Solution was to buy new calipers, always grease the piston seal when the pads are changed, and do not let the pads waer more than 50% before changing them (this prevents the piston from ever protruding very far).