I'll bet a lot of extended families go at that time, making larger capacity vehicles a must-have, and if it's based on supply and demand, that's probably where the prices are based on. I have noticed, though, that the deals usually come out the closer you get to your travel dates.
Now, I haven't travelled over a major holiday, but my last trip in early December, I got the ressie about 5-6 weeks out for the best price since I had been watching, which was over 9 months.