What actually happens when a song is “stuck in your head”?

The actual phenomenon of this is called ‘earworm‘.

Though not concrete, it is said earworms are correlated with music exposure, but could also be triggered by things that remind you of the song (seeing a word that is in the lyrics, hearing notes from the song, feeling an emotion that you associate with the song).

So inside your brain, your short term memory works in 12-18 second cycles. At the conclusion of the cycle, all redundant information gets cleared (familiar settings etc) and your short term memory starts to record again. So, a lot of clever music producers are aware of this cycle, and will make music that gets recorded by our brains neatly during the short term memory cycle, and it just stays there after whatever song you’re listening to concludes.

[removed]

Your brain tries to finish it but may be you dont know all the lyrics or too lazy to sing it till the end. Than you stay in a loop.

Mostly you are “recovered” from your earworm if you listen or sing the song from beginning untill the end.