The official term, or at least the term that shows up in scientific papers, is earworm, or in its original German form, ohrwurm. No one is sure exactly what causes it, but studies suggest things that make it more likely to happen. If you only hear part of a song, your brain has a tendency to try to finish the song. Sometimes listening to the entire song, or even better, singing the entire song, will help get the song out of your head. Songs with silly lyrics, or with unusual or unexpected beat patterns are also very good candidates for hanging around in your head for a few days.

All of this makes the Llama Song a perfect candidate. It has silly lyrics, an unusual beat pattern, and no apparent ending. It just keeps going round and round. With this sort of song, even the name or a few words can start the music in your mind. Llama llama duck.