I'm not sure, I believe it is weather related, and possibly environmental also. I know some cultivars are more prone to production of proliferations, so maybe genetics are involved as well. If you set seed pods on a scape with a proliferation, it helps keep the scape alive longer, and can give the proliferation more time to produce roots. You will not always find root nubs on proliferations, sometimes you have to place them in water, or a moist medium, before roots begin to form. Other times you may find more established roots. Some proliferations will actually bloom while still attached to the mother plant, most do not.