My iPhone screen smashes but I didn’t drop it

I was just sat watching TV my phone was on charged on the coffee table and my friend rang me and I picked my phone up and the entire screen was shattered but I didn’t drop it. The screen protector is perfectly fine and I don’t understand how it happend. It has been untouched for around 30 minutes it is a few years old but I don’t think that should cause it to just spontaneously crack.