I have a theory about them. Here it comes:
When you ask someone what it is they're humming they usually answer: Oh, just a song I heard on the radio this morning. And they go quiet.
And the next time you turn around they're humming it again. I think I have figured out why. It's because we stop listening in the middle of it.

This could be very important! The human mind is amazing and we always seek to fill in gaps. Our vision isn't such that we actually see every spot in our field of vision, but we have passed over them at least once and we recognize the pattern so our brain fills in the gaps "from memory" so to speak.
When you go round humming the song you never get to the end of it. If you would you'd stop humming it, wouldn't you? But you don't.
So I think it's not necessarily the very last song they played on the radio when you left this morning, but the song in which you stopped listening that sticks in your mind, and you try to finish it. But as you didn't hear the entire song you keep humming it to try to get to the end of it.
You will never get there.

So I have one recommendation: Make sure you never stop listening in the middle of a song if you don't want it stuck!
And please, please, for my comfort and that of your fellow human beings always change the song if you feel your mind is drifting in the middle of a very bad one. It would be appreciated by all, I'm sure.

96-10-29