In the context of discussing tevilah without kavanh the gemara also cites a dispute among Tanaim whether shechita requires kavanah or not. The Rashba in Toras haBayis (Sha’ar 1, end of ch 1, page 11a in the middle) builds on the parallel between these cases and writes that just as it is clear from the gemara that if a nidah falls into a lake she has fulfilled the requirement of tevilah, so too if a knife falls off someone’s lap unintentionally and somehow kills an animal with proper shechita, the shechita is valid and the meat is kosher. We find this same idea, says the Rashba, with respect to laws of damages – if one leaves a knife atop a building where it can be blown off by the wind, and it ends up being blown down and hurting a passerby, the one who placed the knife is liable based on the principle of isho m’shum chitzav [a person is liable for arson even though the fire, not he, is the agent of harm because the person is the cause of the damage occurring, just as one who shoots an arrow is liable for the damage caused by the arrow because he is the cause – B”K 23].
The Ra’ah in Bedek haBayis disagrees with this entire line of reasoning. By the case of damages, the upshot is that one is liable for damages even though one is physically not the agent doing the harm. By the case of tevilah there is also no requirement that the mikvah dunk occur through direct human causation. However, writes the Ra’ah, by shechita the Torah demands ‘v’zavachta’, that a human agent be the direct cause of the animal's death. If the knife unintentionally is blown or falls off a person’s lap and kills and animal, the shechita has not occurred through human agency and is invalid.
What exactly is the point of disagreement between the two opinions? It is possible that the Rashba rejects the requirement that shechita occur through human agency. However, there is another possibility as well. R’ Chaim Brisker (on Rambam Hil Shecheinim) raises the question of whether isho m’shum chitzav creates koach gavra or not. IOW: a simple case of mazik would be if one took a sledgehammer and demolished a wall. An arsonist has not come in contact with his neighbor’s wall, yet he is liable for burning it down. Does the Torah mean to tell us that lighting a fire is as much a direct act of damage as taking a sledgehammer to the wall, or is the Torah telling us that even though one has not directly damaged the wall, one is liable for being the cause even indirectly of the wall’s destruction? According to the former understanding, there should be no difference between shechita, which must occur through direct human agency, and any case of damages. According to the latter understanding, it would seem to be a unique rule of torts (or murder – see Tosfos Sanhedrin 78a) that imposes liability even for indirect action, but shechita, which demands direct human agency, cannot occur through isho m’shum chitzav.
(Disclaimer: every time I touch on the sugya of isho m’shum chitzav I feel like it is a black hole that would demand hours of work to properly understand, which I don’t have right now. Hopefully I captured something worthwhile even on this superficial level. See BK 23, Sanhedrin 77, the R’ Chaim in Hil. Shechainim, and the Birchas Shmuel in B"K if you want more.)