While eating, what startles the man?

From the text:

He unbuttoned his jacket and shirt and pulled forth his lunch. The action took no more than a quarter of a minute, yet in that brief moment the numbness touched his bare fingers. He did not put the mitten on, but instead, struck the fingers against his leg. Then he sat down on a snow-covered log to eat. The pain that followed the striking of his fingers against his leg ceased so quickly that he was frightened. He had not had time to take a bite of his lunch.