Bonneville Power may have to shut down wind turbines because too much power is being produced by wind and hydro. If they let excess water flow over spillways this could raise nitrogen levels in the water, harming fish. So instead, they will turn off the wind turbines.
If we had a grid that allowed large amounts of power to be stored, then excess generation capacity would be stored for later use rather than being problematic. The big problem with our grid is that generation always has to match capacity exactly, no power can be stored.