I had pretty much this same issue recently. Didn’t matter if I was playing a game or it was sitting idle. Eventually it stopped turning on. I was able to confirm the PSU as the issue by testing it alone (still wouldn’t power on just jumping the power on pins). Coincidentally mine was also seasonic. It was under warranty so they sent me a replacement and I haven’t had an issue since.
Have you seen the cat take a shit outside the box? How close is the shit to the box? My cat sometimes craps outside the box but it is just due to him not walking far enough into the box before squatting, so his ass is sticking out. The poop ends up just outside the door or smeared on the lip.