Mostly .pid, but some .plist, .logs, & .sym, as well. They come from everywhere in the OS - private/var, users/preferences, etc. Automatically, for sure. Maybe Disk Warrior. I was having a lot of unpredictable OS behavior, used DW every week until I finally bit the bullet and did an A & I. It was a piece of cake compared to a Windows reinstall. I wish I'd done it sooner.
All is back to norbal now, but I need the 400M of space this folder takes up.