Turns out that’s exactly what happened. Tesla service pointed out a tiny chip at the bottom of the windshield, invisible from the inside. The crack apparently spread from there- hence no warranty coverage. Seems like they were open to covering it if there was no chip. So, blast away with the preheating worry free, as that theory of mine missed the mark. Chalk it up to shitty winter roads in wisconsin full of loose rocks getting kicked up with snow and ice. FYI, safelite wanted $1240 for installing a new windshield. Tesla is charging $847.