The subordination of the Son to the Father only became a problem when the initial teaching about Jesus being a mighty spirit creature was abandoned in favour of the new Trinitarian orthodoxy. Martin Werner explain the development.
The Primitive Christian conception of the Messiah as a high angelic being also explains for us the fact, which is of great doctrinal importance, that in the Primitive Christian are there was no sign of any kind of Trinitarian problem or controversy, such as later produced violent conflicts in the Church. The reason for this undoubtedly lay in the fact that, for Primitive Christianity, Christ was, in terms of late-Jewish apocalyptic, a being of the high celestial angel-world, who was created and chosen by God for the task of bringing in, at the end of the ages, against the daimonic-powers of the existing world, the new aeon of the Kingdom of God. Hence there was no ground for any new problem concerning the relationship of Christ to God. On this decisive point, on which everything depends, further clarification is necessary. Because the relationship of Christ to God the Father was conditioned by the direct and essential connection of the concept of the Christ with the doctrine of angels, that relation-ship was understood unequivocally as being one of 'subordination', ie. in the sense of the subordination of Christ to God. Wherever in the New Testament the relationship of Jesus to God, the Father, is brought into consideration, whether with reference to his appearance as a man or to his Messianic status, it is conceived of and represented categorically as subordination. And the most decisive Subordinationist of the New Testament, according to the Synoptic record, was Jesus himself (cf. for example Mk. x, 18; xiii, 32; xiv, 36). This original position, firm and manifest as it was, was able to maintain itself for a long time. 'All the great pre-Nicene theologians repre-sented the subordination of the Logos to God.'' The Trinitarian problem first emerged when the Church in its theology was constrained for certain reasons, which were connected with the process of de-eschatologising, to abandon the concept of subordination for that of coordination. Almost insoluble difficulties then inevitably produced themselves, which in turn necessarily provoked great strife. They concerned, on the one side, the concept of God, and, on the other, the relationship of the new theology to the New Testament as the canon of dogma.
Martin Werner, The Formation of Christian Dogma (1957), pages 124 and 125.
The whole book is well worth reading.