You wouldn't be able to use the active voice here, because the snow (the thing you're talking about) would have to be the object of the verb in active voice. For a different example: Susan's mom tells her, "Go feed the dog!" Susan walks over to the dog bowl to fill it, and sees that it is already full, and the dog is eating. Susan says, "The dog seems to have been fed." Susan doesn't know who fed the dog, so she can't use the active voice. All she knows is that the dog is eating, so the dog has been fed. In your example, we technically don't know who or what blew the snow from the mountain. Realistically, we do - it was the wind - but to be consistent with examples like Susan's dog, the passive voice is used.