Why Brendon Urie play bass in the song "mad as rabbits"?

United States
February 6, 2010 5:18pm CST
I was watching a video where panic at the disco played mad as rabbits on this swedish tv show. I noticed that when they were playing, Brendon Urie was playing bass and Jon Walker was playing guitar. Normally its the opposite though. i was wondering how come brendon plays bass and jon plays guitar in the song?
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