It's from the phrase "big wheel", meaning a person with a lot of power/influence. Similar to "big cheese"... It would have been better to use "cheese" instead of "wheel" IMO.
Pretty sure it's not. I saw something on this topic a few weeks ago but can't quite remember. Iirc, it was a term in an early early OS, where a bit in memory was the privilege but and could be set or unset by turning a real wheel on the computer. This Stück with some people developing UNIX, so they called the wheel group wheel, but none of them are sure who came up with this.