Brief Loss of Situational Awareness
Watching the video several times, it looks like both helicopters were flying normally without any obvious faults. A flock of birds rises from the trees but all appear to be heading away from the aircraft. The nearest helicopter appears to commence a turn to the left and impacts the further helicopter which continued to travel in a more or less straight line until the impact. If the pilot in the nearest helicopter was flying in the right hand seat, there is the possibility that he couldn't see the other helicopter before commencing to turn. Looks like it may just be a case of a brief loss of situational awareness possibly compounded by the passengers and film crew looking for the best angles to film.