Ever seen a PVC fence where one year it is straight and the next it is up and down?
Early in the history of PVC fence, fabricators failed to take into consideration the nature of PVC. PVC expands and contracts with temperature changes (that is the reason vinyl siding must be loose nailed). Over time the fence is subjected to many freeze-thaw cycles. Eventually this causes the post to break free from the concrete footing. After fabricators correctly identified the problem a solution was created. By cutting a large hole in the bottom of the post to allow the concrete to dimple into the post, the post is locked in place.
Some fabricators do not put the vertical pinning into the posts. They excuse themselves of the few additional seconds it takes and place the responsibility for cutting the vertical pin hole on the installer. This requirement is often buried in the installation instructions. Unfortunatley, most installers never receive or read the installation instructions.