Why do you think a cold shiver ran down okonkwo's back as he remembered the last time the old man Ezeudu visited him?

Okonkwo likely shivered because the last time Ezeudu visited him, he predicted that Okonkwo would have a severe loss, which was later confirmed when Okonkwo's gun exploded, accidentally killing Ezeudu's son. This memory would have triggered fear and anxiety in Okonkwo, as it reminded him of his impending misfortune.