I've seen a few related explanations behind the motivation behind using the word "revert" to mean "convert", e.g.,
They seem originate with the ahadith:
No child is born but upon Fitra. He then said. Recite: The nature made by Allah in which He created man, there is no altering of Allah's nature; that is the right religion." -- reported by Abu Huraira, Sahih Muslim 2658 d (sunnah.com)
No babe is born but upon Fitra. It is his parents who make him a Jew or a Christian or a Polytheist. A person said: Allah's Messenger, what is your opinion if they were to die before that (before reaching the age of adolescence when they can distinguish between right and wrong)? He said: It is Allah alone Who knows what they would be doing. -- reported by Abu Huraira, Sahih Muslim 2658 e (sunnah.com)
But it doesn't explain when the word "revert" started being used, or even if the Prophet used an Arabic equivalent himself.
Question: When did converts to Islam start being called "reverts"?
In the comments to How is the notion of a "revert" compatible with not imitating non-Muslims?, G. Bach described it as a "modernist misnomer", which indicates it's new. If it's new, it could might come close to innovation, which is typically frowned upon in Islam.