When I bought my MK4 I did not know it had the active bonnet system. it works if a pedestrian falls on the bonnet at speed, the bonnet will active by the sensors on the front bumper and exploding rams on either side of the bonnet will activate and the bonnet rises at the back hinges. This is to prevent a pedestrians head hitting the engine lump, by creating an air space under the bonnet, as it rises.
The cost to repair the damage caused can now be substantial, and there has been cases of the bonnet activating by faulty sensors, or by just touching the sensor with a bollard when parking. It has then resorted in a legal battle in some cases, who pays for repair, 'new bonnet if rippled and new exploding rams, and reset system'.
Damaged Convertibles were always weighed up after a crash by car insurers, due to possible warping of the chassis and that a new roof would not fit. So its a case of the cost of fixing against the value of the car, and many just get written off. This active bonnet system, that require replacement bonnet and explosive rams can just throw the calculation in favor of writing it off. There are many MK4s now written off.
On a personal note, I always watch when parking that i do not hit the front bumper off anything