Rear and side windows are generally made from tempered glass which has been heat treated to set up residual stresses in the plate which put the interior under compression. This is what causes it to break into small granules rather than long shards.
However, although it has decent tensile strength it is still a brittle material and very notch sensitive. The most likely thing is that there was a pre-existing scratch or crack in the glass and the impact of closing the door was enough to make the whole thing fail.
It is also possible that it had received some significant impact while it was parked and you just didn't notice the damage and closing the door was enough to make the damaged glass collapse completely.
I suspect that the bowing outwards is just a combination of the fact that the normal slight curve is made more obvious by the fact that there is now a big hole in it and the fact that it can't support its own weight anymore exaggerating the curve further.