When it happened to me I'd been in labour about 14 hours and was halfway down my second bottle of gas and air. I was so out of it and relaxed, more than I'd ever been before in my life. I think the combination of that, my breathing through the contractions/pushing and baby's head/body pressing down, caused it.
It was certainly not pre-determined, nor something I'd share with my DS. I'm not weird or strange. It was just one of those things, like emptying your bowels, that can happen in labour.
Pip x