There's a big wind farm I drive by occasionally and sometimes most of the windmills are feathered. Some are turning, so there's clearly wind. I have assumed this is when the demand is low (or maybe negative).

That’s not necessarily true. In general a single windmill is more efficient at pulling energy out of the wind than two are. And the marginal costs of windmills are not zero. I.e. their maintenance cadence (also) scales with active hours. So it might be a “at this price-wind point it’s not profitable for us to run a second mill”