In my opinion, and judging from the verses in the Quran, the group that believed Ezra to be “Son of God” is likely in reaction to the people who started saying Issa (not the Yasu’/Jesus we think of) was “Son of God”. The appellation was for pious people and actually meant to signify “God’s Person/Servent of God” which could have been morphed to it’s literal word-for-word sense.
PS: The Issa v/s Yasu’ debate is quite a lengthy one… to understand my above statements you will first have to read Kamal Salibi’s “Who was Jesus”.
PPS: Also, to note here that by signifying someone as “Son of God” in the sense of being pious, the argument is that this person’s teaching of leading life a certain way (AKA al-Diin) is the CORRECT one and all else is false. Subsequent verses (9:31+) confirm this by mentioning that people have forgotten God and have started to honour their scholars/leaders (hence making them **Partners to God).
PPPS: Yes the evidence is contained within the Quran… the verses poceeding directly after 9:30 give it away very clearly.