> In most cases I've seen so far, contractual agreements with gas, coal or nuclear (who struggle to switch themselves off quickly without hurting themselves) have been the reasons for negative pricing and the grid wasn't actually at 100% renewable at the time of the curtailment. In other words, solar switches itself off, while other, dirtier plants get fined (negative price!) for demanding that they be allowed to still run.

No contracts or fines are necessary. The plant is making the power and dangerous or expensive things will happen if nobody takes it. So they pay for it to be disposed of.