When you run over glass, it can flatten your tire. If the glass is big enough and sharp, it can puncture right through the tire and cause a blowout. Even if the glass doesn't puncture the tire, it can still cause damage that will lead to a flat. The weight of your car pressing down on the glass combined with the speed at which you're going is enough to grind the glass into your tire, wearing it down until it eventually goes flat. So, while running over glass won't always cause an immediate flat, it's definitely not good for your tires and should be avoided if possible.