It was because the wind was blowing and solar was doing well. At 14:30, wind was 15GW and solar 7GW against demand of 28GW. Nuclear was an additional 5.3GW. See

It was because the wind was blowing and solar was doing well. At 14:30, wind was 15GW and solar 7GW against demand of 28GW. Nuclear was an additional 5.3GW. See https://grid.iamkate.com/ https://grid.iamkate.com/. I don't know the specifics of why this results in such negative prices, but it's relatively unusual to have both high solar and wind simultaneously, and it happened at the weekend when demand is lower.