hey man that sucks. at least be thankful that your not injured (im assuming your not since you didn't say you were) and that the damage to the car could have been worse. best of luck repairing your car and yourself if need be
my only possible idea is that the lugs were not tight enough and it hit another car head on but the funny part about it took off after being hit by another car, made a left at the near by light and it was off. a fellow driver who saw it all drove after the rim to get it and brought it back. he said i would have never found it because of the distance it travel but the odd thing was that there wasnt even a scratch on the rim