The Rosh writes (1:1) that shein and regel are patur in reshus harabim because "it is normal". Why not just say that its a gezeiras hakasuv (i mentioned this in an earlier post)? Rosh answers that regel is patur in reshus harabim because it is normal and well accepted that the animal has a right to be there, and it is impossible to demand that the owner follow it at all times. Based on this, the Rosh says that if the animal stepped on a piece of wood in reshus harabim that broke objects on the other end in the reshus hayachid, the owner of the animal is exempt since the animal was walking in an area where it has a right to walk.
This seems to contradict our gemara that talks about "tzroros", when the a pebble flew out from the animal that was walking in the reshus harabim and caused damage in reshus hayachid, we conclude that the owner is liable (chatzi nezek since it is tzroros). The Bach on the Rosh understands that the two rulings, being patur by the long piece of wood, and chayev by the pebble can be reconciled. However, he doesn't really explain the sevara of the distinction.
Perhaps the distinction is that when there is a lapse of time between the cause of the damage and the actual damage, such as when a pebble flies out from the animals feet (the cause occurs seconds prior to the break), we are forced to view the pebble as the "mazik" rather than the animal. Since the "mazik" is in the reshus hanizuk, the owner of the animal who is responsible for causing that damage is chayev. But when the animal steps on along piece of wood and at that moment it breaks vessels on the other end, the cause and effect occur simultaneously. Since there is no lapse of time between the cause and the effect, we can view the animal as the "mazkik. Being that the animal is in reshus harabim, the owner isn't liable. In short, determine the "mazik" as the cause of the damage at the moment of the damage. Therefore, the moment of damage by the pebble is when it smashes the object, and since the "mazik" which is the pebble is in reshus hayachid, the owner is liable. But the moment of damage by the piece of wood is when the animal steps on it, and since the "mazik" is the animal which is in reshus harabim, the owner is exempt.