This happened to me with my first car (2000 Celica GT-S). I got in my car after work one summer day and the mirror had fallen and was sitting on the dash (no wires going into the rear view then). Sure enough it took a chunk of the windshield, similar to your pics, with it.
I was able to get it to hold with some marine grade silicone adhesive until my state inspection was due, at which point I had the windshield replaced. I wish I knew what caused it and sadly I didn't dig deeper into the issue (I was in high school then and this was lower on my priorities). My only guess would be delamination from some sort of manufacturing defect in your glass. Typically, insurance will cover some or all of the cost of the windshield.
Good luck!
OA