To be fair, it appears that the driver of that first-generation ML started it, at least from what the footage shows. It's the ML that first hits the G63, and even after the AMG-powered car retaliates, the driver of the ML goes on to be the more aggressive party.
As for the white Rolls-Royce and second-generation ML (also in the convoy), they were lucky not to get damaged during that epic and ridiculously stupid SUV battle. It's like watching Pacific Rim, but with cars.
So why did this happen? Aside from the fact that the action took place in Russia, there isn't any more info on this incident at this time. Of course, we do know that you have to be a tiny bit off your rocker in order to willingly turn your two tonne+ car into a mobile projectile.
Still, once the driver of the G63 pinned the ML and floored the throttle (45-second mark), the extra muscle of the G-Class really became evident.