Surely the real question to be asked is why a commercial airline was overflying a known warzone? To put passengers's lives at risk just to save a bit of jet fuel strikes me as being crimminally irresponsible.
Whenever a ragtag semi guerilla army has it's hands in the toybox of modern munitions it is a pretty safe bet that events will occurr that should not. To further add to the confusion many varied agendas are being pursued in the area. This makes pointing the finger accurately almost impossible. Perhaps the real truth will emerge several decades down the road.