Glass can store alot of energy in the stresses created as the piece cools. Its entirely dependent on the Glass type/composition and how it's Annealed. Glass is not thermally conductive. When working with Glass its very easy to have a piece that is hard on the outside and molten in the center and the opposite is just as easy when your taking to long between reheats
I'm familiar with Soft Glass,not Boro-silicate or Hard glass so these temps are not for all glass
Melting temp 1750-2150F (954-1177C)
annealing temp - 850-950F (454-510C) highest temp it will maintain a solid form
over 1000F(537C) and it will start to get plastic
Annealing is a slow process that depends on type of glass and thickness it starts @ the temps listed and decreases gradually to ambient temps 1/4"(6mm) it could be fully annealed in 8 or so hours 2"(52mm) 2-3 days maybe longer
Take that 2" piece and allow it to air cool, I would fully expect it to send pieces across the room with much enthusiasm, when it pops. A 1/8" thick vase Will pop when air cooled and send its pieces a few inches this is not to say it will not go further or that each will not just crack. its just that the pops are more typical.
Its not a dropped piece. Its Floor Art. Same as Designer Firewood