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.