There are indeed Holiday rates that Disney uses for Fridays and Saturdays (sometimes Sundays) in December, they are approximately double the normal rate. It is because of supply/demand.
It simply boils down availability of the DJs and what the market will bear. All Disney DJs also perform local non-Disney events as well. Disney wants to make sure they are able to secure their DJ availability against non-Disney events (because of the high demand for Holiday Parties/events).
Hope this helps!