I think I just figured it out. Many of the posts/teposts are triggered by what's being passed around on Facebook that day. It's shared by a few folks whose friends share, whose friends share, etc. I began to suspect it when I did it myself, but knew for certain when an obscure quote a friend posted the other day appeared here as a different meme. Most are old jokes recycled. Heck, I have lists of them on my computer, if I ever draw a blank, but this quote was so obscure that to see two different versions in one day? And my fn friend shared his version a day before.
Take my three submissions today: two are old jokes, one is a variation of an ad campaign. All done in poor taste, and one, because of the wording, is NSFW. Not sure if mods put a hold on them, but two submitted 9 hours ago, the third, an hour or two ago. None featured. Go figure.