What causes monsters to spawn with ridiculously negative-enchanted equipment from time to time? I mean, sure, you see variable enchants on monster equipment usually, but then sometimes there is that one monster that has -4 and -5 on every piece of its equipment. It seems to most often occur on skeletons, but I recently killed an Ur-Khan whose equipment was junk like that.
Skeletons spawn with equipment that is pre-disenchanted by -2 before it's even rolled, so they naturally have a higher chance of having ass equipment. And that's on top of the chance of the equipment being naturally enchanted (or disenchanted).
As for Ur-khan it's reasonably likely he just picked up that ass equipment - due to the way the game code works, enemy AI will always try to equip anything it finds with a higher enchantment than what it currently has - regardless of whether that enchantment is negative or not.