I had a similar problem with my 85 vanagon with the problem getting worse.
It seemed that going over a bumpy road jarred the mechanism unlatching the
door.  Eventually, the vibration of the van, opened the door.

Solution.  Replaced the rear door latching mechanism on the sliding door
itself.  Removed the interior door panel, unscrew the old, screw in the new.
The problem was in loosening the 2 screws due to the bad angle.  I used a
socket wrench with a screw driver drive on the 3/8" rachet (I got this off
my impact driver.)

Took about 30 minutes with all the screwing around.  Got the part from
Camelback VW in Phoenix.  They have an 800 #; it cost me around $30 and they
ups'ed it to me.