As the interior cools and contracts, it pulls on the outer surface, creating a huge amount of thermal stress. Snapping the tail of the drops causes cracking that unleashes the pent-up energy, making the drop explode spectacularly.

What can cause glass to explode?

Spontaneous breakage of tempered glass is most commonly caused by chipped or nicked edges during installation, stress caused by binding in the frame, internal defects such as nickel sulfide inclusions, thermal stresses in the glass, and inadequate thickness to resist high wind loads.

Why does glass explode when heated?

At What Temperature Does Glass 'Break'?

However, high heat (>300°F) and excessive thermal variations can cause glass to shatter or break. Glass is a poor thermal conductor and rapid changes in temperature (roughly 60°F and greater) may create stress fractures in the glass that may eventually crack.