Quite possibly a faulty brake hose.The inside of the hose could be collapsed,or the bracket that clamps around the hose could be clamped too tight,keeping pressure to the caliper.Try opening the bleeder and see what happens,if the rotor turns easy,there`s a problem with the hose.Try the brake again to see if it is still sticking,if it still is, take something to loosen the clamp on the hose and repeat trying the brake to eliminate this part.