That's because of the way the game does pathfinding. When a peep reaches the intersection, he randomly chooses which way to go. It's possible that when reaching an intersection, the computer chooses a direction, but forgets to consider the ride, since it's doing the pathfinding algorigthm first. If you've got it a square back, the person decides to follow that path, runs into the queue, and then considers whether or not to ride a ride.
Jman