Does the Star Trek: The Next Generation Writer's Guide count for you?
The Starfleet reasoning (or "in-universe" reasoning) is that officers need a year of bridge duty before being promoted. I don't remember if that's all officers, or only ones in certain fields.
(And, at this point, Gene Roddenberry was still running the show, just as he was in season one.)
The behind the scenes reasoning was that they didn't think they'd need a Chief Engineer and had gone through three of them in one season and by the second season, they realized they needed one and it was better to use an existing character than a new one.
I suspect part of the reasoning, the second time around, was that they were trying to keep the stories more character-focused and deal less with the technology, but soon found they still needed a chief engineer. (LeVar Burton stated he loved the change, since that meant that when he had a scene, it was moving the plot forward and was more integral, rather than just responding to orders and pushing buttons.)