I see two possibilities here

1) they found something they didn't like. They made a judgement call based on true facts. You need to move along
2) They found something that isn't true. You can probably get this reversed IF the right person looks over it. I might consider putting a suit and tie on and trying to talk to someone in HR face to face. Even if they won't reconsider, finding out what the item was can help you fix it for next go round. If somebody else's felony is showing up on your record, better know that now.