Could it be that Anakin/Vader awoke something that caused Plagueis to awaken and return in the form of a new person.
I think back to this scene from ROTS:
Anakin/Darth Vader is clearly infatuated with the ability to keep friends and loved ones alive.
Maybe Vader starts to look for ways to learn the power in order to help out Padme that Plagueis possessed and unknowingly awakened the soul of Plagueis.
Would also tie in the deleted scene from the movie quite nicely.
Quote:
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“Kylo Ren, I watched the Galactic Empire rise, and then fall. The gullible prattle on about the triumph of truth and justice, of individualism and free will. As if such things were solid and real instead of simple subjective judgments. The historians have it all wrong. It was neither poor strategy nor arrogance that brought down the Empire. You know too well what did.” Ren nodded once. “Sentiment.” “Yes. Such a simple thing. Such a foolish error of judgment. A momentary lapse in an otherwise exemplary life. Had Lord Vader not succumbed to emotion at the crucial moment—had the father killed the son—the Empire would have prevailed. And there would be no threat of Skywalker’s return today.”
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Would not only then be referencing the fact that Vader would not kill his son, but also that Vader looking into a way to control life due to his sentiment for Padme is what causes the soul of Plageuis to return.