> At issue is what happens when too much electricity is on offer. The enormous supply of hydroelectricity during spring runoff can push electricity prices to zero—the BPA gives away electricity to local utilities for free when it’s forced to produce more than it wants to. Since wind producers enjoy production subsidies, they can push rates below zero, effectively paying other utilities to switch off their generators.
The key bit in that passage: effectively paying other utilities to switch off their generators
It is possible to turn off wind turbines (though they get annoyed at this because of clean energy subsidies) or send water over a spillway of a run of the river dam (with some environmental consequences).
There is a neat visualization with that peak demand - https://www.youtube.com/watch?v=jvnaiHFT6nQ - its the visualization of the Columbia river being sent down stream (right to left) through each dam (white bars). The management of "turning on some power for peak noon consumption" begins at 5:00 am as the uppermost dams start releasing more water downstream. So, if the wind picks up in a few hours unexpectedly, the hydro system pays negative (thats at issue in the article) to have the wind power turn off because they can't really stop the river (especially if its a spring high water event in there too).