If your car don't have four wheel disc brakes : the rear brake self adjuster will do this some times , maybe after you have back up and hit your brakes it will tigthen up the rear brake shoes and they drag a bit till they wear down , in this case your emergency brake would only pull a short distance to set , meaning the rear brakes are adjusted way out
if your car has 4 wheel disc brakes : you probably have a caliper sticking from time to time


