Bava Kamma 18: Don't... Excrete Where You Eat

Why are chickens pecking at a rope from which a bucket will drop, if they peck through? Will that make a difference as to how much the owner of the chickens would have to pay? Soumchos says the chicken owner should pay full damages - is that the reason the chicken case would need full damages? Is it just following his opinion? Also, can the case of tzrurot (pebbles) ever become a case of "mu'ad," and therefore entail full damages or not? Is that Soumchos' rationale? What about other cases of potential "tzrurot" - attenuated causation in the case of droppings (dung), which damage. We're left with the question whether the case of the pebbles is closer to "regel" (trampling) or to "keren" (goring)?