Why do oven door glass panels sometimes shatter?
Because oven doors use toughened (tempered) glass, and the property that makes it safe is the same property that makes it fail spectacularly rather than gradually.
How toughened glass is made. The glass is heated then cooled rapidly, so the outer surfaces solidify first and end up in compression while the interior is in tension. That locked-in stress makes it several times stronger than ordinary glass and means it breaks into small blunt granules rather than shards — which is why it is used where safety matters.
Why it fails suddenly. The whole pane is a balanced stress system. If anything releases that tension at any point, the stored energy propagates through the entire pane in milliseconds. It does not crack and stay together; it disintegrates all at once, frequently with a loud bang.
The causes:
Edge or corner damage. The edges are the vulnerable region. A chip, a knock while cleaning, or damage during installation can leave a flaw that eventually gives way — sometimes weeks or months later, which is why failures often seem unprovoked.
Nickel sulphide inclusions. Microscopic impurities introduced during manufacture that slowly change volume over time and can trigger spontaneous failure with no external cause at all. This is a known and accepted phenomenon in toughened glass generally.
Thermal shock. A cold liquid spilled on hot glass, or a wet cloth on a hot door.
Mechanical stress from a distorted frame, over-tightened fixings, or a door slammed repeatedly.
Scratches from abrasive cleaners or scrapers, which create stress concentration points.
What reduces the risk: avoid abrasive pads and metal scrapers on the glass; do not close the door by slamming; let the oven cool before cleaning; and never wedge the door or hang heavy items on the handle.
If it happens: the fragments are blunt but numerous and travel surprisingly far. Isolate the appliance, keep children and pets away, and vacuum thoroughly. It is a replaceable part, and a manufacturer may treat a spontaneous failure as a warranty matter.