Ooh! I also have this (but didn't know the name)!

For me, I'm pretty sure it's related to having just awful seasonal allergies as a child - I'd quite often get stuffed up and have an unfortunate amount of pressure in my sinuses and ears. So then I'd pinch my nose and try to adjust it, eventually realizing I didn't need to pinch my nose.

My friend who does free diving would kill to be able to do it! She can't learn it.