One news report stated a farmer saw the plane fly low above him with "the engine" running. It could have been a single engine failure, which should not have been catastrophic. He may have only said "the engine" because he couldn't tell from the sound if it were one or two engines running.
I'm sure the pilots must have known the terrain.
I would suspect the possibility of a loss of cabin pressure. Procedure for that is to put on oxygen masks, and immediately descend.
Some people don't handle the air above 6,800 feet very well.
If their altimeter wasn't accurate, they could have been much higher,and began suffering symptoms of hypoxia faster. The immediate descent could have done exactly what you said, controlled flight into terrain.
I work for one of the major aerospace companies (one of Sukhoi's competitors with this jet, actually), and this post is very uninformed. Nothing in the reports indicate that it was an engine failure, and if so the pilots probably would've raised a distress signal.
It's also pretty much impossible for it to be a loss of cabin pressure. People handle 6,800 feet perfectly fine - in fact, regular flights are pressurised to an equivalent of 8,000 feet, and you don't see people suffering from hypoxia in most regular flights.
As has been mentioned, this was probably a demonstration flight intended to impress the customers. There was a first flight earlier that day which did the same thing, and when they do that, they turn off the ground proximity warnings to avoid constant alarms while they're performing the pass. However reports indicate that the visibility became much worse for the second flight, and if those proximity warnings have been accidentally left off from the first flight earlier in the day, they may have had no warning of a collision with the mountain. Of course, none of these speculations can be certain until the final report has been released. This is a tragic loss, and my condolences go out to the friends and relatives of the passengers and crew.
