I'm having a hard time wrapping my mind around the whole rig sliding sideways at the same time without a wind or something pushing it. I would think that even if one part slid one way, then there would be a natural bend at the hitch and the other part would be forced in the other direction. Are you absolutely sure this isn't a sudden violent sway that you've been able to correct by steering? Either way, I wouldn't be taking another trip with that rig until figuring it out for sure and getting it fixed. I don't mean to be overly intense here, but definitely get that rig to a reputable dealer for help because it could kill somebody.