I find the incident in Chapter 5 interesting because of the way Mrs. Shears approaches Christopher in anger rather than panic. Christopher has removed the bloody fork from the dog, and is genuinely mourning the loss of its life, all while thinking about how dogs are so much easier to understand and gage than people are. Mrs. Shear's language and tone of voice are frightening to Christopher, therefore he hides and defends himself against what he doesn't understand by covering his ears and placing himself in the fetal position.
I understand this completely, because I've seen it many times (particularly in younger children). Children with autism hate loud noises (school bells, yelling, airplane engines ect.), and placing their hands over their ears is common, as is taking the fetal position. These are sources of comfort and defense.