Glass is a brittle material and therefore it can shatter if it is hit or subjected to a sudden change in temperature. For example, if the glass shower door was hit with something hard, such as a hammer, or if it was suddenly exposed to very hot or very cold water, then it could shatter.

There can be a number of reasons why a glass shower door might shatter, but some of the most common causes include high temperatures, sudden changes in temperature, and impact. For example, if the door is inadvertently struck by something hard (e.g., someone's foot), it could cause the glass to shatter. Additionally, if there's a sudden change in temperature (e.g., from hot water to cold air), it could also cause the glass to shatter.