People actually sell Bitcoins during holiday season in exchange of fiat money for buying needs, wants and gifts for christmas. It is not where they use Bitcoin to buy gifts. Most holders do that in December that is why prices will fall. It could also be that it is some kind of manipulation by whales where they dump huge ammount of their Bitcoin funds to create panic as people would start to fear if the price will continue to fall so they will cut loss and decide to sell and whales will surely benefit from weaker hands by eating their bait.