check for a sticking caliper piston(s),
pads may too tight in their slides,
lastly the caliper may be of the sliding type,usually is if it only contains 1 piston, this sliding part often sticks/seizes.
basically you press the pedal to brake when you loose the pedal the pressure in the brake line drops the disc at this point should turn freely after a couple of seconds if the piston is slow returning the pads stay in contact with the disc longer causing heat build up
wirecutterman, May 2009