I'm mystified by this and hope someone has some ideas to help me out. Here's the scoop.

Bike in question: 2003 era Marin Highway One, a flat bar road bike with mid-grade Shimano components (actually had Ultegra RD).

Sold it to my friend who had a crash a couple weeks later. She is positive there was a loud noise from the rear, after which the crash occurred. The damage included a mangled and destroyed RD (missing pulleys and half the cage) and RD hanger. I replaced them both (went with used but nice condition 105 and new hanger).

Shortly thereafter she complained of shifting problems. Turns out one of the chain links had been bent in the previous crash. Installed a new chain. Everything is working fine.

A week later, another problem. This time, it's another mangled RD and hanger. Pulleys gone, cage half missing.
According to the rider, she was on a relatively flat, paved path, in the middle chainring, and shifting to a smaller cog in the rear when it went out. She said she was "maybe a 5" on an exertion scale of 1 to 10, so not pegging the power meter for sure.

Rear spokes show no signs of damage or misalignment. Rear wheel is true.

I've never seen anyone tear up a rear derailleur like these, and now it's happened twice. What in the world am I missing here? The first time I thought "maybe the jockey pulley screw backed out, the pulley flew out and BANG!" But twice in a row?