It was a bad habit that I had since forever, I didn't know I had it and nobody told me about, even my parents as a kid never brought it up. I was in my mid-twenties until somebody had the guts to let me know and I'm grateful to them. Since then I'm very mindful about it and trying hard not to do it. When I'm in a relationship, I ask my so's to let me know if I start doing it. You'd be surprised how hard it is to unlearn such mechanical habits, but I'm trying my best and it's working but it's an endless battle to not forget and I have bad periods.

Anyway, about the "wtf" part of your question: be a bit more compansionate about it. Yeah, it's disgusting, but judging from my case there are people that don't even know they are doing it and they may have not been told about it.