1) The Torah states, "And he who strikes an animal must pay" (Vayikra 24:21). We learn from this that it is only if one actively strikes someone else's property that one has to pay, while if one damages indirectly he is exempt from payment.

The Yad Ramah (Bava Basra #107) writes that in the scenario in Bava Kama 60a, where he stirred up the fire and the wind also caused the fire to spread, the logic behind the exemption from payment is that he did not do direct immediate damage to the item that was burned. If it would be possible to prevent the fire from spreading one would not be liable at all, because no damage would have been done at the end of the day. This proves that the action he did when he lit the fire was theoretically reversible, which proves that one's damaging action would not inevitably cause damage, so he is exempt from payment because he did not do an act that would definitely cause damage.

2) In fact, generally speaking, indirect action is exempt on Shabbos. The Gemara (Shabbos 120b) cites the verse, "You shall not do any work on Shabbos" (Shemos 20:10), which implies that "doing" is forbidden, while merely "causing" is permitted. If so, why does our Gemara say that when one throws the wheat into the wind, he is liable even though it was the wind that did the Melachah of "Zoreh," of sorting out the good wheat from the chaff?

The Rosh (#11) answers that, indeed, what the person who threw the wheat into the wind did was only a Grama. However, "b'Hachi Chiyevah Torah" -- "this is the way the Torah said one is liable." This is the special nature of the Melachah of "Zoreh" -- that the chief participant in this forbidden act of work on Shabbos is the wind. Winnowing is a unique Melachah in which the wind plays the biggest role. Here, the Torah said explicitly that even though one's act is only indirect, one is liable. In contrast, with all other forbidden work on Shabbos, one is liable only if he did a direct action.

Kol Tuv,

Dovid Bloom