Are you sending out a notification to your existing customer list, maybe by a mailing list or by Twitter? It might be that you do an efficient job of marketing to a strong customer base, so you get a strong surge in sales up front.
I don't send any notifications or have a mailing list. C4S updates their list showing you uploaded though. They will tweet for you too. I do not use twitter or any marketing on my own. I just have years of doing videos under my belt. I have regulars and they are loyal. I'm also regular with uploads. I don't advertise or market myself whatsoever. Even on C4S I don't do anything right now. I think maybe it had a lot to do with being regular and also like mentioned the categories you do as well. Plus probably luck like with any site.