Well known that very often a catastrophe happens when several negative factors occur together. Here, very likely the TOW was at the limit (this will be known soon), also no visibility at night after passing the shoreline. The crew did not flow for a long time before this second leg (Moscow - Sochi is just some 2+ hrs) but did not sleep the whole night (it was 5+ a.m). The plane reportedly started a prescribed right U-turn (likely intensive with significant roll). If at that moment anything happens (e.g malfunction of one of the engines) it may pose a difficult task to the crew to be solved almost instantly.