They aren't necessarily generating it -- sometimes they're just storing it. When we produce too much electricity from wind turbines here in Denmark, Norway buys this at a negative price (to offload the Danish grid), stores it (using pumped hydroelectric energy storage), and sells it back into the market when prices are positive again.
I’m not aware of any pumped hydropower, but we can buy cheap and use less of the stored water when prices are low. The dams have capacity to store water on a seasonal timescale. However with shifting weather patterns and more renewables these margins may become strained. This winter we may have to import not only to handle peak loads, but also because some dams may run dry.
