sgtrock21:
Did you ever flip tiddlywinks? Not sure the same thing applies…
If the manhole cover got slightly dislodged so it was sitting over the hole with about an inch or two of gap, the (rear) wheel of a vehicle could cause it to flip as it pivoted around the two points where the cover touched the manhole. If the gap were towards the front of the vehicle, it could clear the vehicle and flip upwards. At least that is my theory.