Based on the amount of distance between take-off and this point....they’d probably reached near 30,000 ft or more. So, a manpad missile won’t be in the final answer. It can only be four possible answers: massive engine failure, another aircraft or drone hitting it, a long-range missile, or a bomb onboard. It wouldn’t surprise me if it was a drone.
Doubtful a drone at 30k+ feet.
The BBC news report mentioned there had been previous tail damage from a bad landing, but the Russians insist the damage had been repaired. If they are wrong, a malfunction in the tail could perhaps have caused the aircraft to dive until the stresses exceeded the design specifications and disintegrated the aircraft with a separation of the tail section.
Could be structural failure. Or electrical failure - it is a fly by wire plane.