Why is the appearance of wolves related to the saying, Ashes to ashes and dust to dust?
Since the phrase "ashes to ashes..." does not appear at any point in the story, I assume that you are just asking about this in general.
The phrase that you mention is a paraphrase from the Christian Bible, specifically from Genesis 3:19. There, God is telling Adam and Eve that they will die and return to the dust (which is what they were made from in the first place). The paraphrase of this is often used in funeral ceremonies and things like that.
Once you realize this, you can see the connection. Now that the wolves are coming, Georg and Ulrich are going (we assume) to die. I suppose you can take it one step further and say that they are going to go back into the natural cycle because they are actually going to get eaten by the wolves and so they will enter nature more directly than most dead people do.