Maybe it was a combination of everybody seeing "Crash" and some refusing to watch "Brokeback Mountain." But I think the main reason "Crash" pulled off the upset is because its subject matter - racism, fear, and intolerance on the streets of L.A. - hit Academy members where most of them live.
Many critics from other parts of the country found the movie's criss-crossing characters and storylines unrealistic, and indeed they are - if you're taking the film literally. "Crash's" director, co-writer, and co-producer Paul Haggis intended it as an allegorical snapshot of a city in the midst of demographic change, and as an L.A. native, I can tell you that's how most of us saw it, too.
http://www.nydailynews.com/entertainment/story/397276p-336749c.html
