Glass is shaped at a high temperature while the glass is still plastic.
As it cools and contracts, stresses build up in the structure.
Large important pieces of glasswork are annealed - that is, they are cooled very slowly over a period of several days.
This is not cost-effective with everyday objects such as drinking glasses, so virtually all household (and pub) glasses are highly stressed.
It just needs two events to trigger a catastrophic failure.
i) A local weakness or scratch to act as a focus for the stress. These are bound to occur quite frequently in regularly handled items.
ii) The trigger for the collapse is a further stress that acts at the previously formed flaw. That stress is caused by expansion, and that generated by the heat from your hands as you pick up the glass.
Maybe you should be claiming danger-money from your employer?