Prices go negative because it's basically a grid control service at that point. You have a load you need to have taken, and are willing to pay someone to help you manage the grid.
It's the other way around; prices go negative because you have too much power, not enough load, and you're paying someone to take that power because you can't curtail like solar or wind (perhaps you're a gas turbine or coal/nuclear generator that must run for frequency control purposes).