Electrical outlet arcs only for non polarized grounded plug
When you "bumped" the grounded cord, you bent up the grounding clamp of the outlet and made momentary contact with the neutral side of the outlet causing the arcing. When you removed the grounded plug and just used a two prong plug the ground inside the outlet didn't make contact because no ground plug was inserted.... Those outlets are very close quarters inside and can arc over with little pressure when damaged. I'd turn off the breaker for that outlet until it gets replaced.