The spot price of electricity is largely determined by the market dynamics of supply and demand. Lots of things affect supply and demand, including the availability of different types of generation.
For example, when there is abundant supply – there is lots of wind, which can't be stored so has to go straight into the grid; or bountiful stores of water at the hydro stations - prices tend to be at their lowest.
On the other hand, when there is heavy demand that can't be met from renewables, generators run coal and occasionally diesel. They're expensive to run so the market price goes up.