Could be to do with how high the prices are. It may be a stereotype, but I think clubs like Leeds attract a more working class crowd who get more passionate and excited, hence the atmosphere at Elland Road. With the cheapest tickets weighing in at around ã40, the most expensive at close to ã100, and the cheapest season ticket at around ã900, you're probably going to end up with lots of the more "orderly" supporters who are happy to cheer a goal, but more likely to go silent, or even boo players, when the team is struggling or when nothing's happening.
As Jip has pointed out, we sometimes have the same problem at United, but it doesn't get as bad as often cos the ticket prices are only about half as much as Arsenal's.