Here is a "story" that I heard about what happened, I can't vouch for its reliability. It is different than I was expecting.
Someone at the party had recently stolen a large amount of money from drug dealers. A group of the drug dealers went to the party looking for the thief. The thief went out to the parking lot to talk to the drug dealers, which went about like you would expect. The thief ran back into the party, and was followed by the drug dealers…. slaughter ensued.