With me, it's not the tune that makes the earworm, it's that the song has a word in it which is connected to something I am doing.

For example, if I am making grape jelly, the line about "He is trampling out the vintage where the grapes of wrath are stored" will be an earworm that day.

I wonder if earworms could be connected to allergies. When allergies are bad, the brain and spine are under excess fluid pressure. Could the pressure be irritating the part of the brain where the repeating tune is stored, causing it to "play" the piece again and again? I think this would be a great idea for a study.