I think it's common, yeah. I'm applying too and I've noticed that a LOT of programs have deadlines in December - usually December 1st or December 15th. I used to look at med school apps (changed my mind, of course) and their deadlines are all during December and January, too. Same with a lot of other grad programs, as well (neuroscience, microbiology, etc), so it's not just an MPH thing, either.
I'm not quite sure what a proper timeline is for applying but the best thing to do is get everything in ASAP. I'm planning to get recommendations in September and have everything sent in by the middle of October! Good luck with your apps!