by regular tzruros, the chicken kicks up pebbles which directly damage the dough or whatever. by the braisa with the bucket though, the chicken breaks the rope and its not the rope that damages the bucket, but the floor that damages the bucket. the bucket breaking is twice removed from the chicken, why isnt it considred koach kocho?
T. Bacon, Fair Lawn, USA
Ko'ach Kocho refers to a pebble that the animal kicks, which, in turn, "kicks" another pebble (i.e. dislodges another pebble). That is, an object that has been set into motion by the animal ("Kocho") ("Kocho") sets another object into motion (Ko'ach of Kocho). That is not the case here.
However, you are correct in differentiating between the bucket and the normal case of Tzeroros, in that the bucket breaking is more removed from the animal (the chicken) than the normal case of Tzeroros. In the very next line, Rava suggests that the owner of the chicken should be Pature in such a case since "Basar Tevar Mana Azlina." Based on this very bucket Beraisa, the Gemara concludes that evidently, the Halachah of Tzeroros applies even in such a case, where the damage is more removed from the chicken.