I guess I know why the spike at the close on Friday. You don't suppose word of OBL being killed was leaked to anyone, do you? ETA: Now I am reading that it happened Sunday, I thought I had seen where it was a few days ago.
Banks and floor traders can do it. And when their traders choose to abandon their options, it affects retail guys. If you are long then be sure on what your brokers policy is. If it's unclear then cut the position.
From the "latest official" report I read, Obama was signing the order just before market close on Friday.
Banks, market makers, floor traders generally try to stay delta flat. If on expiry they find themselves long or short delta they might sub optimally exercise options to reduce that exposure going into Monday. As a short options holder you run this risk even if your option is 10-15 cents out of the money on expiry day.