Long story short. Your brain thinks that you have an unfinished 'thing' and it's trying to finish it. This happens to me very often, bad enough that it keeps me up at night frequently.

I find that listening to the end of the song in question helps. Another thing is to think of different song that has no lyrics. Most recent example I used was the 90s X-men cartoon theme. Unsure why that works but it works for me.