Yeah, I think the hypothesis is that your brain is trying to avoid muscle fatigue by pedaling faster. The "lower resistance/higher reps" idea. I think we all know that leg sensation that says to us "hey, this is getting hard, downshift now please".

Other than sprinting, I find my power starts to drop off as I approach 100 rpm on a flat grade. But on uphills, maintaining power above 90 rpm is difficult for me, with the "sweet spot" 80-85.

I vaguely remember a study of elite cyclists that found their efficiency and power was unchanged between 80 and 100 rpm, but they both dropped off above 100.