What precipitates Johnny's rampage at the fast-food restaurant in the novel, Johnny the Homicidal Maniac: Director’s Cut?

While Johnny is eating a taco, a woman walks by and remarks that his appearance is "wacky," and that she would just die if her son looked as wacky as Johnny. Johnny comments that he doesn't leave home to be ridiculed; other people get involved in the conversation, and Johnny becomes enraged, killing them all.