I've had that happen, let the pads wear too thin and pistons tend to stick in the caliper, once new pads are in no more binding after, freeing the pistons that is.Took the wheel off this evening and undid the caliper. Didn't look too bad but pads were worn through.
Swapped pads for some old ones cadged off MrGorsky. Strangely I had new front ones in stock but no rear ones; clearly I was planning on looking at the brakes last summer and never got around to it. Or maybe did but front didn't need doing.
Wirebrushed everything and then noticed the bottom slide that although the bottom slide pin went in easily it wasn't as bouncy as the top one and didn't bounce out as much. Swapped it for an old one I had kept that felt better (rubber not so hard) and reassembled everything. Took it for a test drive and all fine.
Not sure if they were worn through and it was binding because past its usual range of motion or because slide pin was slightly stiff. Bit of a mystery.
Better order more parts for the weekend...