Purely anecdotally, as a pet owner this doesn't surprise me at all. I have two rescue cats that, from the descriptions from the fosters, had very stressful lives before I got them (attacks from dogs, being attacked by other cats in a feral cat colony, etc). I've also cats that had a very relaxed prior life. The differences in their behaviors is stark; one of the rescues displays a lot of stress behavior whenever there are loud noises or anything surprising. One of them has calmed down a lot, but those behaviors still pop up pretty regularly.

I don't know enough about animal psychology or their inner worlds to say, "that's PTSD," (I do think, mind, that animals experience emotions, but how that experiences compares to human emotion, I don't know) but there sure seems to be something about reliving trauma there.

Astounding that we’d think otherwise.