The only reason I've ever seen given for Wills supporting us is when he said "It's kind of in the middle of the country, isn't it".
Good enough for me, your Royal Highness!
One can always Google, but I think you'll find that William's mentor (kind of a life coach) was a Villa fan so, as he had no other footballing influence (and as he said he didn't want to pick an 'obvious' club), he's always followed Villa as well. I think it's a lot more than Cameron's so-called following.