explain this phenomenon...

I was just getting up from the lunch table when Caroline starts singing a line from "Super Trouper"--the very song that was in my head.  Then Sophie exclaims, "Hey, I was just thinking of that song!"  And Isabella pipes in, "Me, too!"  And the thing is, no one was humming the tune out loud and we hadn't played our ABBA Gold album in, like, at least a week!