Before talking about explosives lets first look at something much more likely that happened before - say like Air France flight 447 quite recently, a jetliner full of people plummets from high cruise alt. to sea. Or numerous business jets that fell from their cruising altitudes after penetrating a thunderstorm (a Westwind jet out of Dallas executive airport is a particularly nasty case), so yes, it had happened before that a jet aircraft fell from their cruising altitudes and there were no explosives involved. By the way a structural failure often happens during a way down when pilots try to recover and overstress the aircraft - this was the case for the Westwind scenario - overstress caused the gear door to separate which flew off and severed the horizontal stabilizer. Overstressing has little to do with old/new aircraft.
This is one post that makes a lot of sense. Overstressing (or even structural failure) has little to do with old/new aircraft