From previous posts, it appears that the main tail section was found a distance away from the main crash site. This suggests that the tail section broke apart from the main body.
SAMs of this class typically have a proximity fuse, i.e. it doesn't need to contact the aircraft to explode and damage the aircraft. The 777 is a large aircraft. There is a high chance that the missile exploded near the tail of the aircraft sufficiently to cut through the tail section or at least enough for the tail section to separate. The aircraft would then lose it's ability to sustain flight and fall to earth. The lack of a flaming decent / smoke trail points supports this theory, i.e. the main fuel tanks were intact and not ignited upon missile impact/explosion.