What could cause a motherboard to short circuit?

What you could be experiencing is is a short from the bottom part of the motherboard and one of the screws standoff in the computer case. First remove the mother board and notice all of the screws holes and examine the reverse side of the motherboard for scratches around these holes. if you find any use an insulator ( plastic or cardboard washer) on the underside of the hole where it meets the case srews stand-offs. Also check for screws already in place in these stand-offs that might be shorting the motherboard.