How very very sad. And unavoidable too.
IMO I observe the lower a/c being below, back and to the right of the higher a/c. It then climbs and banks into the overtaken a/c. This is a blind spot (albeit transiently) for the lower pilot.
Nothing whatsoever to do with smoke, wind or birds. I would wager a years salary that the lower pilot lost SA.
RiP.