There are several facts. Jesus is God. God the Father is God also. Jesus is One with the Father. There is only one God. Jesus' mother is called the Mother of God since she gave birth to God. Since Jesus is One with God the Father, and since there is only one God. Mary must be the mother of the Father as well as God can not be separated but is one.
You still haven't laid it out in the form of an argument. Here you go:
(1) Jesus is God
(2) The Father is God
(3) Jesus and the Father are one.
(4) There is only one God
(5) Mary is Jesus's mother.
(6) Mary is the mother of God.
Therefore,
(7) Mary is the mother of the Father. [from (2), (4),(6)]
But the conclusion does not follow. That is because there are three Persons in God. Mary is the mother of God because she is the mother of the Second Person of the Trinity. She is not the mother of the Father or the Holy Spirit. And yet, because the Second Person of the Trinity is God, and because Mary is the mother of the Second Person of the Trinity, it is nevertheless true that Mary is therefore the mother of God.
-A8