It is simple supply and demand. Electricity generation at any given point in time must be sufficient to meet demand for electricity, otherwise blackouts occur. When you introduce intermittent generation sources into the market, like solar and wind, it becomes more difficult for this matching to occur as generation output is weather dependent. Consequently, you get times in the wholesale market where a confluence of factors momentarily drive up wholesale prices: wind stops blowing, sun stops shining and demand spikes due to it being the middle of a hot summer day (e.g. middle of the day in January, which is summer in Australia, so everyone turns on their A/C). Prices shoot up until it entices other generators to make up the shortfall (e.g. gas turbine operators).