Oil spills only happen when oil is accidentally tipped into the ocean. But if there is a war going on the enemies might try to spill oil into the ocean.
My opinion: People were careless and not paying attention.Technical: it is still being investigated but what they think happened is a safety device called a blowout preventer failed to stop the flow...
This depends on what oil spill is being referred to: See related questions: To be honest really they "said" that it started off with an cement job when the oil rig exploded. The Exxon Valdez Captain...