Got my new windshield installed yesterday at Safelite for $338 total. The estimate from the dealer was $840. Everything seems OK so far with the Safelite installation (knock on wood).
Regards to what causes it, the Safelite guy said that Mazdas, Toyotas, Hondas, etc. allow more flex in the frame, and it is this added flex that causes extra strain on the windshield glass; and hence cracks from even the slightest of pits in the glass, especially when it occurs near the edge. So I would opine that it is probably a combination of the added flex and making the glass too thin that is at the root of the problem.
The dealer did say that he would have run mine through as a warranty issue, but he found just the slightest of a small pit on the crack, about a half inch from the top edge of the glass. Nevertheless, I still don't think that such a miniscule little pin head size pit should be the cause of a such a significant window crack.