I'm with Tim Leaning who posted above, also touched on by Kedall.
For me it's not the song, it is the moment in time and the atmoshphere.
It happens when the audience are in especially good voice and everything is going really well, then you hit a word or a phrase which just kills you on the spot. It "fortunately" often happens to me during the chorus of the songs, near the end and I can fake a bit and re-compose myself for the next verse.
Bloody Magic, JohnB.