Maybe 1 reason is because more newbies are actually reading already posted threads, that might have answers to some questions they'd normally start their own discussions for in the past. I know I've noticed quite a few posts from newbies within ongoing threads in about the last 2-3 months anyway.
Another might be because the FDA Deeming garbage doesn't have much new going on lately. There was a huge increase in posts because of it, from about last May-June through even Nov.-Dec. it seemed.
Or, I've noticed this on other forums, too - When the weather starts getting nicer in various places, and more people are spending more time outside,...the # of posts/amount of activity overall usually drops.