Do you have pets or children? Perhaps one or both walked across, sat on, sat something on top of the Kindle. You don't carry it in a pocket do you? Do you keep it in a cover or case/sleeve? Have you dropped it? Did the Kindle undergo extreme temperature changes in a short amount of time?

Lot of questions. I've owned 8 Kindles over the years and several Sony readers as well. I've never had a screen break, but I'm very careful with my Kindles. The Kindles do have a glass substrate and it can be broken if you or a pet or a child is too rough with it, but under normal conditions that should not happen.