[Frozen_skidmark]

I had this happen on a Nissan sentra the edge of the glass was chipped. Which is why it cracked. They didn't warranty it though and fought me on everything else.... Sigh! Haven't even looked at a Nissan since.
I would take a pic before the crack travels and go show them, see what they say. If there's no rock impact, then it has to be a defect on the edge of the glass. Stand your ground if they back peddle, and start asking for names and write them down. People start thinking twice once you have their name, because you're not just naming the dealership when you phone corporate.