Derailleur tension pulley thread damaged by vibration?
It was almost certainly due to repeated removal and re-torquing. Aluminum threads on lightweight bike parts just give up the ghost sometimes in that way even when not mishandled per se.

Plastic bushing-type derailer pulleys really don't much attention in terms of lubrication. I put in a drop of lube every once in a while just to keep them quiet.

One could probably helicoil the stripped cage, but I'm obsessed with helicoiling things :).