Because they're blind and don't really know what dancing looks like, so they just move to how the music makes them feel, I assume. I don't know any blind people personally. My family is deaf though so I guess it's just like how deaf people talk oddly and might give their kids names that don't really sound very pleasing.
I don't know but I'll make an assumption. I used to do this thing as a kid where I would see how far I could walk or run with my eyes closed without ramming into anything. For some reason, I would reflexively lower my head or do a side to side weaving motion; I had this intense feeling that I was about to hit my head against something, and wanted to avoid it. Maybe blind people feel like that a lot.
