I am with Wageslave at post#3.
I would believe it if the Russians were to admit that an explosive device had detonated.
Failing that I would guess at a fuel-air explosion. That might have been a secondary result of an engine break-up of course.
It is not clear to me if the explosion or the departure from the runway came first.
If runway departure came first, the wing (and therefore fuel tanks or fuel lines) might have been disrupted by the collapse of the port undercarriage. However the wings can be seen (at 27 secs elapsed) in the light of the explosion to be absolutely level.
The port wing is completely broken, up not backwards, by the explosion. It is almost inconceivable that the aircraft design would allow a powerplant failure to have that effect.