This is just a guess, but the concepts of Buddhism and other Indian religions might shed some light on the ontology of this series' world. In this context, perhaps the most accurate term would be "suffering" (the most common translation of the term dukkha) rather than "pain". According to Buddhism, life in the material world is permeated with suffering born from our dissatisfaction, attachments, and desires, and is repeated in a cycle of rebirth (samsara); breaking free from such cycle is the final goal. Perhaps the knockers are naturally attracted to places with people exhibiting feelings that lead to suffering, and seek to liberate them from their material bodies (the vessels they speak of) in order to stop it. Fushi agitates them more so than usual, being the embodiment of unending dukkha not just for himself but also for the souls he contains within himself, both preventing their rebirth and their liberation from rebirth. However, this doesn't explain why the souls stolen from Fushi can be recaptured—is it because the way the knockers "store" them is different (similar to a state of nirvana) while being a part of Fushi forces them back into the world of suffering?