Unless something has failed or not been used correctly, a Reese slider CANNOT pop out of either position.
There is an "over-center" spring inside the driver's side tower that locks the release handle in either maneuver or travel position. The dog on the end of the release lever arm then drops into the corresponding hole in the slider rail and is locked. You can make it stop at dead center straight up, but once it starts to either side the spring forces it to the end of travel position and it locks.
1. The over-center spring is broken or has come loose inside the tower.
2. The release handle was not fully locked in the travel position in the first place. (I know! I know! You would never do such a thing, but it happens and would explain your problem.)
I have never heard of a broken spring but that, too, is possible.