Lots of reasons! Mostly, it’s related to supply and demand, so that when demand for power is high and supply is stretched, spot prices are usually higher, and when demand drops and supply levels even out again, spot prices tend to move downwards too.
But it ain’t a simple industry (sorry folks!), so there’s also a number of other factors at play here, too. Weather conditions and temperatures, the availability of renewable generation, and any outages or problems with our electricity infrastructure can also influence where spot prices sit.