It's a ps/2 keyboard. When I pushed it in to seat it, then mobo slid about a cm and just shut down. When I took the mobo out, I found a factory standoff was inserted near the I/O, but not at a screw hole. I'm thinking that slight movement was enough for that standoff to short the mobo. I'm just glad it didn't fry it.
I moved the standoff and snugged each screw a bit more. I guess I was afraid of cracking the board by tightening them too much. It doesn't move now though.