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Barrack in British English can mean to jeer or boo someone. That's the opposite of root. I've never heard the word barrack used like this in America.
As a verb, it means "cheer" - they are cheering for him.
Plenty of other meanings use as a noun, but that's not how it's used here
Also the somewhat old fashioned phrase “Up the [magpies or other team]” also would have the opposite meaning in the US than it does in Aus. In fact I hear the US meaning more and more, probably due to TV