[franga2000]

This isn't exactly my field of expertise, but I'd say it's almost certainly over-production. You can't "throw away" electricity you generate and you can't change production rate fast enough in many types of plants (like coal, which is most commonly in the negative on this chart). So if it's very sunny and windy for example, renewable plants will generate enough power for the current demand, meaning the coal plant should wind down. But since that would take many hours, plus many more to warm back up later, the plant would incur a significant loss in potential profit once their power is needed but they can't provide it. So it sometimes makes more sense for them to pay "the market" to take away their excess electricity, so they can keep running at a constant rate and be ready to sell power for high prices when nobody else can supply it (during overcast windless days or at night).

As for what "paying the market" means, it might be an incentive for companies to use more (by paying for and extra shift of workers to run a factory), or to cover the losses variable providers will incur when they wind down in order to prevent grid collapse. The latter is probably more of a factor than the former, but I don't have practical experience in this field to be able to say - it probably varies with time and location.

[cinntaile]

You can turn off solar and wind and by doing that you don't have to throw away electricity. It seems more likely that they're subsidized meaning they can drop their prices below zero yet still make a profit.