I don't know how they compute this list, but at least briefly two questions from Math.StackExchange were on the list of the hotest questions in all of StackExchange: http://stackexchange.com/ . :-)
There has always been 1 to 2 questions on the SE front page from each site, it's not that surprising.
See the question How do the “arbitrary hotness points” work on the new Stack Exchange home page? on MSO on how the hotness is determined.