In short - it was cheaper to pay people to take power than it was to shut down generators.
This is what happens when daytime power consumption is 20-30% higher than in the middle of the night.
If only we could consume power more evenly...?
Most people immediately think - "why don't we store the power in batteries!". I suggest - do the math. The energy to be store is gargantuan!!! Basically there are only two way to store that much energy:
- Thermal potential
- Gravitational potential
Thermal potential is easy enough to visualize - heat something up to several times the boiling point of water and insulate it until it is need 8 hours from now in the day. Thermal solar power is one example of this. This is why thermal solar power is more attractive for base electrical demand - it can still generate power when the clouds come in.
Gravitational potential does not involve a black hole or a TARDIS - but water. Using power from base load generators at night to run pumps to lift water a few 100ft up a hill or precipice into a large (kilometers wide) reservoir. The gravitational potential of 1 million tonnes of water is far greater than $100 million in batteries. And at a fraction the cost.