It was a celebration move Rodgers invented early in his career called 'the championship belt', likely a corny reference to a WWF championship belt. Every time he scored a TD, he would make a very demonstrative championship belt holding move as if to say "I'm the champ, bitch!" It didn't really need to be explained, it was hilarious and it worked.
A couple years after he started using it, Rodgers began appearing in State Farm ads. At the time State Farm was promoting a "discount double check" feature to always find savings for customers. To promote it, they had several sketches with Aaron Rodgers where State Farm customers "stole" Rodgers TD celebration and redubbed it the "discount double check."
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The reason why was unclear, but Rodgers plays the part well, seeming annoyed that his awesome move has been stolen from him. The joke is taken further when the people who steal his move do not recognize who he is.
Similar to the
infamous Peyton Manning nationwide ad, the sellout is so extreme that it actually works as comedy, and took on a life of its own. In the same way that people now mock Peyton Manning
by slamming him with verses sung to the nationwide jingle, the "championship belt" celebration by Rodgers has been renamed "the discount double check" by fans, both mockingly and affectionately.