I agree with everybody. These are all possible. To add two more, hot spots on a drum (these can be there from new) can do it. Sometimes turning the drum takes care of that. Sometimes the hot spots are too deep and the drum has to be replaced. Had one on the 5th and it would like lock really hard. New drum and no more lock. If it only locks when wet, might be sticky ebrake cable. Cable gets rusty and doesn't release all the way, can make brake touchy. Take the oil can and every chance you get, put a couple drops of oil on the cables. That might free them. Now I've made your day by giving you more work.