One time I saw that people were getting targeted promotions and I did not. So, I called the Plat line (this was before the merger), and they registered me for it.
I have not received a promotion this time, and haven't called them either.
By the way, I'm Lifetime Gold, currently Titanium, and have been either Plat or Titanium for five years now.
So, unless they're going after people who don't stay that much, I think it's totally random. Hey, why bother rewarding your best customers...