Windshield heat is one of the factors that give windshields strength. Many planes have speed restrictions if the heat is inop. Why? Bird strikes. I don't have an explanation why those windshields broke, but if they were heated at the time I can't imagine it was just gusts, or gusts putting on more stress than birds. I once hit a Golden Eagle square on the windshield of a 727 at 250 Kts. The aluminum across the top of the window was pealed back and the shock broke every filament in every bulb on the panel. We had to relamp the gear lights before landing. The windshield never cracked or broke, they're pretty strong.