I read an article a couple of years ago on this, and found it fascinating! I wish I could find it. I'll look for it later and see. It basically was studying these occurrences, and found that when people get text messages, their body releases endorphins/seratonin. When they looked at these people getting the phantom buzzes, it was almost as if it was their body's new response in cheering them up!
Basically, it was your body trying to knock you out of a funk.... you started feeling a little low, so your brain sent you a buzz that had worked in the past for cheering you up!
It was an interesting article.