can't really explain why it pop, but we have the same problem with cheapy bulbs here (mainly the candle shape ones from
sainsburys -- my "discount" homebase ones have no problem
). the filament burns out, and for some reason the glass pops out of the bayonet fitting (poor adhesive, maybe?)
so, this entails digging around in the cupboard to turn off the RCD for the lighting ring, and then using a pair of pliers to get the metal bit out... most inconvenient