People getting end of year bonuses, or corporate budgets that must be used in full before the end of the year. Also people maybe donating in someone else's name as a Xmas gift.

i tought christmas donations

Every organization sees a donation spike in December. You'll notice that most of them do their annual campaign around that time, to help boost it even further. People are in the giving spirit around the holidays or something.

I dunno but its cool, it is tradition now.