Lots of possibilities to consider.
A. the plane crash was an accident. Least likely possibility in my opinion.
B. It is Prig doing a ninja vanish as he got word that being near a window as in his future, so it was time to bail and live a quiet life in Africa. Not likely but possible. Wouldn't put it past the slippery bastard.
C. Payback by the Generals affected by the purge. Possible. Proof will be seeing what machinations occur in the internal politics of Russian
D. It was the Ukrainians. Highly possible, IMO.
E. Putin ordered the hit. A bomb or Russian AD bought the plane down. It will be reported as an accident.
I am leaning between D & E, with E topping the list. Putin is hearing people mutter about him being weak and the failed moon landing, which could of buoyed up his standing, sunk that chance. He senses the knives bring drawn and was forced to act.