door locks 96 explorer

I have a 96 eddie bauer and the doors lock when the car starts to move but when I put it into park they dont unlock. the front doors I just pull on the handle and it unlocks but the back doors stay locked until I hit the unlock button what could cause this?

I don't think they automatically unlock on that year. My 97 has never unlock when I put it into park. If I open the drivers door only that door unlocks. I have to hit the unlock button to unlock all the doors and it has always been that way.