Comment by jeroenhd

The bottles were sold as "drink and food" bottles, but expiring/fermenting food turns the food bottle into a pressure vessel.

I was initially surprised too, because I mostly know Thermos from their coffee/water/etc bottles, but apparently they're also selling these with the intention of storing perishable goods, and in that case a pressure relief system of some kind is a necessity.

Often bottles have special threads with holes in them to let out the pressure when you twist them open, but it appears they didn't do that here.