This happened the 8th of september during election season with planes flying for the SP
and the CDA political parties. For ease of reference I shall refer to the planes as SP1 (banner 1), the photoship (SP2) and CDA (banner plane that got hit).
The photoship was trying to make photos of SP1 flying at roughly the same altitude to his right. The CDA plane was flying a bit lower to the right of the photo plane.
Autorities are still investigating so we will have to await an offical verdict. From the movies it is clear the pilot of the photoplane was facing the sun making it harder to see the CDA plane flying a bit lower and slightly to the right. The photographer probably did not notice the CDA plane either as he was preoccupied with SP1. In the second movie linked below you can see the SP1 pilot pointing to the CDA plane. This warning might have been misinterpreted by the photoplane as a request to go down to get photos from another angle. This however is purely speculation on my part!
Here's some more footage, including footage of the two planes seperating at low altitude over the beach (1st vid) and footage from inside the cockpit (2nd vid)
The planes involved where a Husky (PH-PHX
/ CDA) which made a succesful emergency landing on the beach near Wassenaar. The photoship was a Cessna 172 (PH-SBM), it made a succesful emergency landing at Rotterdam airport. The other plane (PH-LRM
/ SP1) also landed at Rotterdam.
[Edited 2012-09-30 15:35:14]