My guess it would be related to gas. It's my understanding that new utility-grade wind and solar installs have to have generating capacity equal to or greater than the nominal generation capacity of wind/solar to maintain a stable base load when the wind isn't blowing or the sun isn't shining and that natural gas powered two-stage plants tend to be the "insurance" layer (they are relatively clean, start fast, can throttle up/down far more flexibly than other modes of generation...)