This term comes from the town of Canton in Central Illinois where people have a tendancy, due to life in such a provincial area, to miss high-brow humor and explain simple jokes with what they see as...jokes.
Example of Cantoning:
person "A": "This room smells like 15 kinds of ass."
person "B": "Yeah, almost like 20 kinds of ass!"
Person "A": "What's green and flies?"
Person "B": "Superpickle!"
Canton person: "Yeah, hahaha, or super green bell pepper."
*everyone pauses and stares, looking awkwardly at one another*
Canton person: "Sorry, I work at Hy-Vee."