Tailgate of 2003 Honda Odyssey locks within two seconds of power unlock. Why?

When you use either the key-chain remote or the lock/unlock toggle inside the car to unlock the tailgate of our 2003 Honda Odyssey it immediately re-locks itself within about 1.5 seconds. Using the key to unlock it manually works. Has anyone else had this problem and/or know what's wrong and how to fix it?