There are times where the rooftop solar and base load generation for a utility is greater than the total electricity demand. Demand and supply for the grid have to be balanced, or else you will end up tripping plants and/or damaging equipment. Since they can't "turn off" rooftop and most grid scale solar generation, and they can't really turn off and on their base load plants throughout the day, their best option is to try to sell the excess electricity on the wholesale market. The problem is that the other utilities are in the same situation, and they need to offload electricity. Since no one wants the electricity, wholesale prices will actually go negative. Basically a utility will pay another utility to turn off a base load plant and take the excess electricity from them.

This is why grid scale energy storage is such a hyped technology - if you can store that energy when supply is way too high and dispatch it when renewable generation is low (no wind/solar), you can maintain high renewable percentage in the generation mix. Without it, you can only really go up to a certain percentage of renewables.