One eyewitness:
David Ocampo said: "One of the helicopters sounded as if it was firing off shots or experiencing small explosions that were completely out of place.
There were about five metres between the two of them, but the one in front seemed to stop and that caught my attention. 'The other one smashed into it from behind with its propellor after trying to swerve to avoid it and fell to the ground and exploded. The earth shook when they fell.
I rushed towards it to see if there were any survivors but it was just a ball of flame. 'I couldn't see anything really, just smoke and the tail of one of the two helicopters between the bushes."
Moment helicopters carrying French Dropped stars smash into each other killing ten | Daily Mail Online
So perhaps the first helicopter had mechanical difficulty and the second ran into it. "Five metres between the two of them"? Unbelievably close.