The problem with closing to submissions in December is the flood of subs that arrive in January.
These days, we normally get an average of about 220 submissions in a given month (though there's a fair bit of variation). Last January we got nearly 330; this January is on track to be about the same. It's fewer total subs than we would've received if we'd stayed open in December, but they're compacted into a shorter period, so it's a significantly higher reading load.
The other thing we've had more of than usual lately is people submitting stories that have already appeared online in one venue or another. Sometimes it's on their personal web pages, sometimes in their blogs, sometimes on publicly accessible sites for writers. Sadly, we can't consider unsolicited submissions of stories that have been published before, and a story appearing on the web counts as publishing as far as we're concerned. (I gather that most other publishers agree with that these days.) Moral: if you want to publish your work in a magazine, don't post it in a publicly accessible place online. Unfortunately, if you've already posted it, we can't consider it even if you take it down before submitting it.
I was going to say more, but I gotta run.