For cpp6f and vortex:
Not sure I'm the right guy to say this, but I got here first so here goes.
The airplane was flying in 'coffin corner'. This is a strange part of the flight envelope where stall speed and the maximum mach number converge. The stall speed is a constant indicated airspeed, based on 'q' or 'half rho vee squared'. And this is the problem, rho, the air density, decreases with altitude so the true ground speed for stall progressively increases. At the same time the speed of sound in air decreases with lower density and so with higher altitude.
At the flight level they were flying, I think it was fl370 or 37,000ft, the margin between airplane stall speed and structure maximum mach number was only 25 knots. Go slower and you stall. Go faster and you disintegrate. Don't know your airspeed and you sure as heck will do one or the other.
I hope this is a valid explanation, others greater than I will come after and will surely correct me.