What event triggers Mitty's courtroom daydream in "The Secret Life of Walter Mitty"?like what makes that daydream occur

"A newsboy went by shouting something about the Waterbury trial" is what triggers Walter Mitty's daydream.  Always forgetting things and being belittled by his wife for this forgetfulness, Mitty fantasizes that he is "the greatest pistol shot in the world" on trial as he suddenly remembers the puppy biscuits.