Basically, George Babbitt experiences what many people still experience today: a mid-life crisis. In his case, it is sparked by the imprisonment of his best friend. After Paul goes to jail, Babbitt begins his rebellion. He goes to the movies when he should be at work, he flirts with women (and eventually has an affair), and goes against the conservative principles of the other businessmen in his social circle.
However, while Paul going to jail may be the straw that broke the camel's back, Babbitt was not really happy in his life before that. He is unfullfilled, as shown in the dinner party scene where he can't wait for his guests to leave. His marriage brings him no contentment either. All of these are reasons for his rebellion against the rules by which he has lived his life.