Likely scenario is aerodynamic break-up....a not uncommon cause in any category of aviation...from shuttles to gliders.
At high speed, if stability is lost and needing correction [obviously], it's common for stresses to exceed airframe capabilities/design parameters.
One theory to ponder, however....'bird-strike', where the 'bird' is some bit of flotsam of space junk [satelite] in a decaying orbit of its own.
Of course, I'd not feel too amused if I were one of the 3 still on the station.
I'd expect they are going through an equivalent 'hell' at the moment...
