RF brake locked up. Why?

You know the story: Everything is new, but not everything is necessarily good... Since they're drums,and I'm assuming everything bled out fine- (would SORT OF rule out rubber line collapse) I would make DARN sure you've got them adjusted out as far as necessary, because if the pistons have to travel too far out in the brake cylinders, they can actually **** and jam at the end of the bore, locking the brake. Now that I think of it, make sure your self-adjusters are on their correct sides! If not, they could actually change as you drive, causing gradual loss of braking, and then the piston hanging up...
Or a wrong (RHT/LHT) adjuster on the wrong side.