If I didn’t put the snow there then it is not my Bor Bereshus Harabim. Moreover, even an actual Bor, if I made it in Reshus Harabim, it is not mine and I would not be liable if not for the fact that the Torah made it as mine — concerning liabilities.
Actually, getting rid of the snow is the city’s job. They put fines on homeowners to get them to do their job. You can’t sue a homeowneer who didn’t shovel his snow, although you can sue him if he did shovel it and caused it to become slippery. I heard of a law professor who purposely doesn’t touch his snow. He says it is worth the fine not to get sued.
In Halacha, using the Svara that is Mechayev the Baal Hakerem for the Hezek since it was his responsibility to move back, perhaps the homeowner is Mechuyav in the Hezek once it became his job to do it.