I had a similar problem on a 1981 Honda 70 ATC, when I took the brakes apart to see why this was happening the brake shoe pad had started to separate from the shoe base and was causing it to bind, I guess water got into the brakes and it could not get out because the drain was clogged with mud and it loosened the glue that holds the shoe pad to the shoe base....be prepared for a bit of a battle to get the drum off.