Aleš Lombergar

Glass can be under tension internally, and remain whole for years, and a slight scratch in the right place can cause explosive crack propogation. As soon as the energy available in propagating (continuing) the crack is more than it takes to form the edges of the crack(s)
an explosion can take place.

Glass, especially glass used in dishes can be poor quality, like aforementioed, but also are tempered. Tempering is a heat treating process that makes the glass stronger but more brittle. Glass in autos is all tempered, per law so it will break into small pieces rather than jagged shards. Tempered glass is also called safety glass for this reason. While it can withstand constant loading, tempered glass has trouble with impacts.

Was this “hair holder” a hot curling iron? Applying heat to most solids will cause them to expand. Because of the brittle nature of a glass, UNEVEN heating will exert pressure on the cooler parts, and when it finally lets go… boom, it goes everywhere. But some glasses are “poorly made” like that other aswerer said, and were cooled unevenly at the time of manufacture, when the the thing went from a molten liquid to a solid. Such a glass may be under pressure always, so that when it breaks, it’s going to go everywhere no matter what.