It's not something I'd heard of before, though I can't say I'd ever given it much thought before doing research for the blog! I picked it up in a couple of articles. One noted that the bare origin story, culled from interpretations of the Bible, wasn't enough so
... storytellers often added a preface relating the circumstances of their [the angels'] expulsion from heaven. Here the passage in the Old Testament was the only source. This story might be told in various ways, e.g., once when God had left His throne for a while one of the Angels of Pride took His seat and looked out over the world. God took this as an insult and was so angry that he started throwing Lucifer and his followers out through the gates of heaven. This went on for a while until somebody, the archangel Michael or Jesus Christ, said that God ought to stop or He would end by leaving heaven quite empty. God then shut the gates and said: 'Let them be where they are,' and there they still remain.
Some Notes on the Fairies and the Fairy Faith by Reidar Th. Christiansen (1971-3)
The second one specifically refers to these being angels who didn't take sides:
The "religious view" might be Christian, occultist, or pagan in coloration, but it assumed that the elfin races were spiritual beings or that
their origins lay in religion. The simplest forms of this belief had long been held by the "folk" themselves. All over England, Ireland,
Scotland, and Wales investigators found local people who thought that the fairies were a portion of the fallen angels or those trapped
on the earth when God locked the gates of hell and heaven; to many they were angels who would not choose either to "stand" or "fall."
On the Origin of Fairies: Victorians, Romantics and Folk Beliefs by Carole Silver (1986)
Apparently the question as to what would become of them at the Last Judgement exercised minds as well, and whether the fallen angels would be readmitted to Heaven at that point, which also generated various stories.