It could have been pushed off the right pivot by something trapped between the bed and the tailgate.
The tailgate pivots on to two circular (donut shaped) pivots mounted on the sides of the tailgate opening just below the taillights. The left pivot completely encloses the left side tailgate pivot pin, but the pivot on the right has an opening near the top to allow the tailgate to be lifted up and out of the pivot when the tailgate is partially opened. Once you lift the partially opened tailgate out of the right side pivot, you can pull the right side of the gate back and to the right and it will slip out of the left pivot and the tailgate can be removed.
It also important to keep the taillgate locked because thieves can steal the tailgate if they can open it. As an added measure of security, you can tighten a stainless steel hose clamp around the right side pivot donut and tighten the clamp screw. Be sure to position the clamp screw so that it cannot be accessed when the tailgate is closed. This blocks the opening in the pivot donut and will prevent the tailgate from lifting up out of the pivot, and most thieves will not hang around long enough to figure out why the tailgate will not pop out, or bother trying to unscrew or cut the hose clamp.
...Rich