Since both back rotors where hot it means the calipers where on. Are you sure it was not the emergency brake sticking? Best to check that system out as well when your into it. For example. If you never use the emergency brake and the garage did it may have stuck.

If you eliminate the emergency brake the master cylinder becomes suspect. One of the internal rubbers is covering the internal port so the rear brakes stay on after you release the pedal. The brake fluid is blocked from returning. The overheated rotors and pads where just the end effect not the cause. I would inspect the rotors and pads you now have. They may still be good and safely usable.