I read somewhere the choppers were flying close but within limits, and there was a sudden gust of wind that blew one onto the other. These were light choppers after all. It's pretty windy this time of the year here in Phoenix.
The only way to keep aircraft from colliding is to have them fly different altitudes. It is normal for helicopters to agree on an altitude for each.
Either someone wasn't paying attention or one was trying to muscle the other to get a particular angle.