Flowering is an unpredictable process in Sempervivum; they can produce vegetative offsets (chicks) every year, so they don’t need to flower to reproduce new seedlings annually. But, from time to time, they do send up a spike that eventually goes to seed; a flowering rosette is at the end of its life; any rosette that does flower and go to seed will die. Think of it as the plant’s swan song at the end of its life as it makes way for a new generation. Any form of stress can induce flowering; drastic changes in light, temperature, and water can threaten a rosette’s survival and thus make it divert its resources into producing the next generation of seedlings. All Sempervivum will eventually bloom after several years of growth,whether induced by stress or simply age. Signs that a rosette is preparing to flower include a closing in of the central leaves, stretching of the main stem, and tilting of the whole rosette.
If you don’t want to remove a dead rosette, you can cut out the impending bloom at the centre and, if you do it early enough, you may prompt some regrowth in the centre.
I’ve never bothered, I think the blooms are weirdly interesting.