At higher altitudes, there is a a thinner margin between stall speed and mach buffet. I have read that the U-2 routinely operated right at the top of its ceiling, so that the margin between stall speed and mach buffet was only 5 to 10 knots. Here is an article that discusses the "Coffin Corner," the thin margin between stall speed and mach buffet at higher altitudes:

http://airplanepilot.blogspot.com/2007/01/coffin-corner.html

The blog post discusses the importance of pilots training for high altitude stalls. Interesting that Airbus has already advised pilots not to lower their speed too much while approaching turbulence at higher altitudes. One does have to wonder if this was a combination of some type of fly by wire failure combined with a high altitude stall, and the pilots didn't have a enough time to reboot the system in turbulent conditions.

BTW, an interesting factoid. According to news reports, since airlines started flying Europe to South America and vice-versa in 1947, this is reportedly the first commercial airliner crash (of course there have been crashes on other transatlantic routes).