The most common cause of an automatically locking door is a bent interior lever. The exterior handle moves a lever that locks and unlocks the door. After heavy use, the lever can bend, causing the door to lock automatically. To fix this:
Remove the three screws securing the interior door lock assembly with a star-bit screwdriver.
Swing the interior door lock assembly out of the way. It will swivel on the inside door handle shaft.
Use a pair of pliers to bend the bent lever up into the proper position.
Put the inner and outer assemblies back in place, and replace the three screws.
Continue reading for a more in depth explanation of this process, as well as my experience with an automatically locking RV door.