Engine is Generac GT990 on 12 KW air cooled standby generator

Upon engine disassembly, I found a fist size clump of metal in the bottom of the crankcase. It was the cover of the oil pump. It apparently was torn loose from the main oil pump body and bounced around in the crankcase. Scar marks are on one of the connecting rods and a quarter inch square is missing from the bottom of the piston and the bottom edge of piston is slightly bent and was rubbing against the cylinder wall. Fortunately no grooves were worn in the cylinder wall. I did find rivets which indicate that the cover was initially in place on the pump. Several of the plastic gear teeth on the oil pump are partially severed but are still in place.

Any ideas as to why/how the cover was torn loose? I am leery of just installing a new oil pump for fear that the same thing will happen again.
Did you notice anything odd before it failed? I have a 4456 12 kw, same engine as yours, and I get a low oil pressure light when its below freezing. I'm curious if you noticed anything similar.