I think the concern is that Jesus is God from
before His incarnation. The following quote I found is from an ex-Catholic, but I think it states the position clearly and succinctly:
No one disputes the fact that Mary is the mother of the human Jesus even though she was not the “supplier” of His human soul. Nor is there any question that the man Christ Jesus was created human in body, soul and spirit. What is disputed is the extension of the title “mother” to a divine nature that eternally existed and was not created in the womb of the virgin. A mother is only the mother of what originates within her womb. The second person of the blessed trinity did not originate in Mary’s body. He is without beginning – has always existed – and has no mother. . . Conclusion: Jesus Christ the man is the son of Mary. The Second Person of the Trinity is her God, not her son, for He did not originate in her womb.
contenderministries.org/Catholicism/marymother.php
Many Protestants take no issue with the term mother of God because there is a use and sense of that phrase that simply emphasizes the deity of Christ. However, others see it as part of a package of Marian devotion that oversteps scriptural warrant. Here is an excerpt from Philip Schaff’s History of the Christian Church, volume III, chapter VII:
It is perfectly natural, nay, essential, to sound religious feeling, to associate with Mary the fairest traits of maidenly and maternal character, and to revere her as the highest model of female purity, love, and piety. From her example issues a silent blessing upon all generations, and her name and memory are, and ever will be, inseparable from the holiest mysteries and benefits of faith. For this reason her name is even wrought into the Apostles’ Creed, in the simple and chaste words: “Conceived by the Holy Ghost, born of the Virgin Mary.”
The Catholic church, however, both Latin and Greek, did not stop with this. After the middle of the fourth century it overstepped the wholesome Biblical limit, and transformed the “mother of the Lord” into a mother of God, the “humble handmaid of the Lord” into a queen of heaven, the “highly favored” into a dispenser of favors, the “blessed among women” into an intercessor above all women, nay, we may almost say, the redeemed daughter of fallen Adam, who is nowhere in Holy Scripture excepted from the universal sinfulness, into a sinlessly holy co-redeemer.
ccel.org/s/schaff/history/3_ch07.htm
