Much as I don't understand how it is that every morning I wake up, I have some random music rattling about my head, I don't understand how new ones end up getting featured throughout the day. I mean, in some cases, it's because I accidentally heard an earwig. But, in most cases, it's completely random. With wakeups, it's generally a song I've not heard in ages (and didn't even ever particularly like). Even more bizarrely, it's rarely associated with whatever it was I was dreaming just before waking up. In the rare cases that I was dreaming it, the music is usually what wakes me up.
At any rate, I've got another random tune stuck in my head and it got there unprompted and in the middle of the day. All I can do, at this point, is think, "Argh: how did I get Symphony of Destruction stuck in my head??"