Does anyone know what causes rear brake drums to warp? I will add that the previous owner of my truck cut the seized emergency brake line. Could this be causing the problem?

 

Over adjustment or a seized wheel cylinder could have caused sticky brakes, then they would overheat and warp. There could also be seized calipers up front causing the rears to take on most of the work.

If there was excessive runout at the last brake job and they were not machined, it may have got much worse.

As far as simply cutting the seized cable, it may have been seized partially applied, again causing heat buildup inside and warpage. Sometimes people cut the cable thinking it is seized when it is not, rather something it's attached to is not functioning.

Also check the shoes to ensure they are sitting even on the backing plate. If the drums were machined with too coarse of a cut on the lathe, the shoes will follow the grooves like a cheap record player, causing a quiver effect over time and clunky brakes.

These are easy to find, common causes.

 

lol i have full discs.