Some people get songs stuck in their heads, I get words or phrases looping over and over. Sometimes I'll get both a song and a word repeating in separate loops at the same time! I think it's a similar mechanism though, it's a lack of closure. I believe they repeat because my brain is looking for connections, or trying to learn something from it. The annoying ones are when I can't remember the ending of a song, so the middle keeps looping over and over. Or if I can't quite remember the definition of a word, very annoying. The best ones are the ones I can remember all of the tune but it surprises me every time.

Edited for clarity.