I had this happen once. The trunk lock actuator was not properly aligned and vibration from closing the trunk lid or slamming a door would trip it to activate. I was able to diagnose by turning the lock with the key and noting it actuated the lock very close to vertical and quite far from vertical to un-lock. I just re-centered the lock actuator so the key rotation before actuation was equal in both directions. The mounting screws were a bit loose so it had moved over by itself.
Another possibility is a broken wire in the trunk harness shorting to the lock circuit.
HTH,
Anthony