Why or how it happens, I have no idea, but I salute it when it drops in. More than once I've been hearing a song in my head, over and over, turn on the radio, and there it is, right at the part I'm thinking about. More than once (in the days of TV you could watch without paying for it) some part of the brain was playing out a more memorable clip of something from MASH or "Frasier" and I'd turn on the TV--and there would be a rerun playing, of just that episode, and just that place in the episode that had been running in my head.
You encounter a really unusual word or phrase, and for the next week or so it seems to be everywhere.
Someone says, idly, "whatever happened to..." and names an old friend who has somehow dropped out of sight. The next day Old Friend appears, walking toward you. Or you see them at a party a week later, they tell you they just came back to town for a few days...
Easy enough to write all of this off as coincidence, but I don't think it is. It's being where you need to be to have something 'familiar' happen and having the good grace to recognize it for what it is. I often wonder at all the times that were missed because we turned right instead of left, or decided that day to take one street to work instead of another.
The mind is a strange and wondrous place, some days it's like looking through a keyhole, getting only glimpses of what's going on in there. Some days the view widens a bit.