Rosstafari Daaaang. /X

In flight breakups aren't entirely uncommon; all it takes is an airplane being put beyond its performance capability. You'll hear about it most often in some general aviation pilot trying to be a hotshot in his trainer -- anything that's preceded by "Watch this!" is never a good sign. There's a reason that they don't barrel roll Cessnas at air shows...

Anyway, you put enough stress on the airframe and it will break apart under the load. There are different ways that it can be done -- exceeding load limit factor as above (in other words, too many G's at too high an airspeed), flutter (like what happened to the Tacoma Narrows bridge)... other reasons too that I don't understand well enough to write about and not sound like I'm making stuff up.

Of course, if it's anything like the above-mentioned TWA Flight 800, it may not have been an airframe failure at all. Few airliners will reach a flight load that would cause a catastrophic in-flight breakup, just because you'd have to be very purposeful in an attempt to reach it.

Anyway, that's part of the reason that they piece together plane wreckage after crashes, to figure stuff like that out... although I wouldn't count of them being able to recover enough wreckage from such a remote crash site to do that very effectively this time.