Derailleur tension pulley thread damaged by vibration? I have been riding my RD-5800 this week with a slightly misaligned hanger. It had some difficult shifting, but it was ridable. 

So the hanger alignment tool arrived today, and I decided to clean the derailleur and re-grease the jockey wheels after aligning the hanger. To my incredible surprise, when trying to re-tighten the tension pulley screw, the aluminium forming the thread broke off from the inner plate. I have now a useless derailleur.

I am trying to establish the probable cause of the problem to prevent a similar scenario in the future. Firstly, I would like to precise that the bolt has never been subjected to more than 4 N.m (Shimano max = 5 N.m), so I don't think that is the problem.

Every 2-3 weeks, I remove the jockey wheels to re-grease the bushings. I have been doing that for the last 8 months.

Do you think that the frequent removal of the jockey wheels caused the problem, or it may be due to the vibration experienced by the misaligned hanger?

Follow-up: I have temporarily replaced the bolt with a M5 longer one and a nut on the other side. It is not the better solution, since it is threaded in the bushing portion, but it will last until my new derailleur arrives.
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 :).