Warped drums -
-- Caused by putting it on the axle off-center and it wears in a non concentric circle. (That's why sometimes you can find the position it was in.)
Since often the wheel holds the drum on, when changing a tire the drum can shift a little, then tightening the lug nuts holds the drum out of position, then it wears non concentric.
-- Also caused by not centering it properly on a lathe when refinishing it - which Autozone and those discount places are famous for! Would you have a tooth pulled by a hamburger slinger?
Take it to a real automotive machine shop - same price, but they'll do it right.
-- Can be caused by putting on the E-brake tight when the drum's real hot - as it cools down the E-brake warps it.
-- Poor quality/thin Chinese drums - available at AutoChina.