Do you mean the rear brakes are locking when they get warm??
Not knowing what year bike you have, what kind of riding you are doing, or how much time is on your bike I would suggest the following.
Take the rear caliper off of the carrier clean the entire assembly to remove any debris that might be stuck where the pads ride in the caliper. Clean and lubricate with high temp grease, the pins on which the caliper rides on the carrier so that it can slide freely.
Make sure that the retaining pin for the brake pads is not bent, and is free from nicks or gouges so that the pads can move freely on it.
Flush and Bleed the rear brake system.
Hope this helps,