Comment by petertodd

Baseload power stations sometimes over-produce on longer timescales than just a few seconds because they'd rather not turn them off for maintenance/operational reasons. E.g. imagine you have a big biomass boiler feeding a steam turbine. Turning it off for an hour or two means everything cools down, which is a thermal stress, reducing lifetime compared to keeping it at constant power.

But yes, certainly poorly managed solar/wind that doesn't have good mechanisms to turn off in response to lack of demand is mainly the issue. In the future, when control systems are better, I'm sure negative pricing will be much less common.