In the lore vampires usually enter torpor voluntarily when their unlives are turning to crap or they have lost their purpose. Mithras did suffer a major crisis after his crazy girlfriend Kemintiri corrupted most of his court. She got banished, the courtmembers got terminated, he committed himself to a mental institution. Or they do it for tactical reasons, to outlast their enemies, fake their destruction etc.

In this mod entering torpor greatly reduces your stress, so NPCs might do that when they reach stress level 3. Some seem to do it when they turn into wights.