it is a form of reproduction for btas, ritteris, and maxi minis, along with most pest anemones.
It is not necessarily caused by stress every time, but stress will greatly increase the chances of it occurring, because the anemone stands a better chance to survive if there are more of them.
Alternatively, excellent health and excellent conditions along with frequent feeding will increase the chances as well, because the anemone senses the environment would be beneficial for it's clones.