Tempered glass, or heat tempered glass, is a safety feature for glass. Tempered or safety glass is required and wise when the glass is in a door or is within 2 feet of a door, and several other applications. It is designed to shatter into safe little pebbles of glass and not the dagger-like chards of float glass.
Your tempered glass failed correctly and safely. Tempered glass can have a stress point where a small amount of pressure can cause the glass to break. I suspect you glass failed because of misalignment, improper racking, or some sort of incorrect pressure, which is common for storm doors after 11 years.
We suspect your storm door is ready for a check-up and new glass replacement. Make sure to once again use tempered or safety glass in the door for your safety.
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