My suspicion is this... the car has been in a rear end collision... the body shop did a lousy job that the latch was only partially engaged so during an atmospheric pressure change, the trunk popped open.
When you drive on highway, the pressure is negative... suction. When you come to a stop, the pressure go from negative to normal... pressure change. Slamming the door is positive pressure... pressurizing interior and force out the trunk.
If the car has not been rear ended, then you have to check your vents in the trunk area... this allows pressure change and vents it out...