In most cases there are no easy answers to the question why a disaster happened. Usually there is a chain of malfunctions, wrong doings by the crew, misinterpretations or just bad circumstances that lead into disaster. As I am sitting in front of a computer and do not have any resources at my hand, I can only tell you what I recall from memory. For an in depth report consult the book Air Disaster Volume II by Macarthur Job. The Lauda flight was en route from Bangkog to Vienna when the thrust reverser of one of the engines inadvertently deployed (which it of course should only do when the oleo struts of the landing touch the runway). At the moment I don't know why this malfunction happened (consult the book), but it caused the wing to stall and the aircraft to violently swirl around its longitudinal axis on its back before it went into a steep descend. The crew had no chance to recover the aircraft from this attitude and I think the aircraft must have disintegrated because of too much stress on the airframe.
Boeing had tested the recovery of an aircraft with a deployed thrust reverser and had found it to be possible. But this was carried out in a height which was as far as I know not above 10-15.000 feet. Aerodynamic conditions at a heigher altitude however are different, as this accident showed.
Sorry, this is all I know at the moment.