Fossil fuels have input costs, but wind & solar have zero or nearly zero operating cost, and shutting them down incurs cost. So while fossil fuel is hurting, they are happy to sell power for $0 for a few hours if that's what it takes to get someone to take it off their hands.
IMO it basically looks like what you'd expect to see in a market in need of storage operations to perform arbitrage & smooth the price curve.
When you have a power "glut" you generally aren't decommiting many resources unless your commitment and forecast were insanely off. What happens is that the more expensive resources are sent to their minimum and are therefore no longer setting price. At that point, the cheapest units (wind and solar) are then setting price. No energy market in the US has logic to say "I don't need anymore and will then just go to zero". The linear programming problem is still looking at the cost to balance power and load (put simplistically).
Storage will help solve some issues, but will likely not be the short-term panacea people are making it out to be.
Here's an excerpt:
> There are several reasons why conventional power station operators, which are either losing money or at least losing profit during times of negative prices, keep their plants running (See study by Energy Brainpool, page 4-5 and the 2016 results from Consentec). They can be technical, for example the power plant can be too inflexible to change its output, or the ramping or costs for shutting down and starting up can be too expensive. Another reason for keeping the plant running can be the obligation to provide contracted balancing power to keep the grid stable or provide re-dispatch power. Alternatively, it may be that a certain production has to be kept up to provide heat for a town household heating network. Those plant operators which have already sold their power at the longer term futures market face no extra costs when they let their units run – they are merely losing the profit that they could make by buying cheap power to supply their customers instead of producing their own.
If the government didn't subsidize renewables when the grid is oversupplied, then other resources would pay them anyway. Prices would even go negative as transients would still exist. Since the subsidies are far less than the premium made by fossil peaker plants, renewable resources are still the first to turn off and the time difference is negligible.
Not to mention that in most countries the subsidies are by FAR the least distorting legislation. Most often (eg the US) renewables get first bid on contracts. That means they are guaranteed to sell their electricity even if the price goes negative; the onus is on fossil plants to turn off. This is a dumb, possibly unnecessary bit of rule. It ensures renewables are used, but it means they aren't well utilized as peaker resources. Renewables are orders of magnitude better for the grid than any other type of generation, but if they wanted they would never be used for anything but baseload.