This link is specifically for real-time market pricing. This is a marginal rate. Presumably, power companies locked in their pricing using the day-ahead rates

This link is specifically for real-time market pricing. This is a marginal rate. Presumably, power companies locked in their pricing using the day-ahead rates yesterday according to their forecasts and risk models. Now there is a surplus of energy production, and it actually makes sense to pay companies to take that electricity out of the grid. You can see the current day-ahead rates by selecting DAM-SPP from the Select Data dropdown at the bottom. The negative marginal rates is an interesting phenomenon, of course, but the real-time marginal rates alone don't show the whole picture.