solar production is just so much cheaper to dial down than gas (whose plants can't be turned down and up again without huge expense).
Sorry, you've got that backwards -- "turning down" solar is exactly as easy as turning down the sun, while gas turbines are quick to ramp up and down. Pretty much only hydro is faster. Historically that fast response time was used meet demand peaks during the day, particularly in areas that use a lot of air-conditioning, because traditionally baseload sources like coal and nuclear cannot easily be ramped up and down. In other words, before gas became cheap enough to use 24/7, its main selling point was how fast it could be ramped up and down.*
All of that said, negative electricity prices, which is what you have when somebody is paying you to take their electricity, are as much -- if not more -- a result of market deregulation and inefficient power grids as they are of the success of renewables. Because of deregulation, utilities and grid operators no longer have total control over the power system, and this means they have to manage an energy mix that is not what they themselves would have selected. Policies giving preference to particular energy sources play a role too. Not surprisingly, there are both up- and downsides to this.
The relevance here is the following: Electricity prices go negative when the grid is getting too much power from inflexible power sources, meaning sources that are expensive to turn on and off (like solar, wind, coal, or nuclear). The power companies look at the situation and evaluate how much it would cost to turn off one or more of their inflexible sources. If it costs less to pay somebody else to take the excess electricity then they do it. That is what happened here: California had an excess of solar power, it would cost more to shut plants down than it did to pay Arizona to take the electricity, so they paid Arizona. It's not a common situation, but it isn't rare, either. The only time it seems to make the news is when it's caused by renewables, but it has been happening since long before there any renewable power sources on the grid.
*I've oversimplified a bit, there are ways to reduce the output of a solar farm, but they are all equivalent to throwing away free fuel (which is what makes it "expensive" and therefore inflexible), and some coal and nuclear plants can ramped up and down much faster than they could in the past, but still not nearly as fast as gas turbines. And despite an increase in the use of gas for 24-hour power, in practice the ratio of energy it generates to that that it could have generated if running 24/7 -- the capacity factor -- is not too different from that of solar or wind.