That's only twenty-odd years after Mary was first officially hailed as Theotokos (God-bearing), at a synod held in
Rome in 430. The title
had been growing in popularity, but many theologians were unhappy, insisting, for instance, that Mary should also be called Anthropotokos (Man-bearing) to balance it, or that she should be called Christotokos (Christ-Bearing). Nestorius, the Patriarch of
Constantinople, preferred Theodochos, God-receiving. He took the view that the divine and human elements in
Christ remained separate, and that Mary was the mother of Jesus' humanity, but not of
his divinity, which rather ruled out Theotokos. He was opposed by Cyril of
Alexandria, a rather unscrupulous character who resented the influence of
Constantinople, and held that the divine and human natures were merged in
Christ, hence the synod, which he called to discredit Nestorius.
From what little I know about him, he was rather strict about keeping women in their place, and quarreled with
Pulcheria. He was condemned as a heretic at the First Council of
Ephesus in 431, but
his followers flourished in
Persia, and the Nestorian
church at its height reached as far as
China. Since
Pulcheria was so powerful, you can see why she'd have been keen to emphasise Marian devotion!