Good news and bad news.
I now know exactly what killed by PixRacer.
The bad news is that I know that because I killed another PixRacer :-/
So it turns out that there was some damage I hadn't noticed on my 250. A signal wire to one of the ESCs frayed and could be intermittently shorted to the (3S) power line. First time it was probably some thick grass pushing against the wire. Latest time was me fat-fingering the situation.
So: PixRacer's really don't like 12V on their ESC signal lines. I think I can forgive the design that!
@Gunaveh Suggest you might try checking for anything bodgy in terms of the RC-Out wiring.
@dipspb Thanks for the offer! I don't think anything but replacing the stm32 is going to fix this problem, 'though; @pkocmoud could probably confirm that.
Lesson learnt: pay more attention after smashing things into the ground at high speed!