The rear door has nothing to do with the alarm.

This happens to me also. I unlock the door with the remote, not thinking, then open the back and start loading the groceries, then beep, they lock up again before I am even finished.

For some reason this seems to happen most often when it is pouring rain.

This is usually followed closely by me mumbling a certain word that rhymes with duck, as I fumble for the keys again.

I guess the relock feature is another clue that Isuzu knew it was easy to accidentally unlock your doors, as I believe it doesn't work if you just get out of the car and leave, but don't lock the door.

It only relocks when you have unlocked the door, but not opened it within 30 seconds.