Why might a soldier who spent time in combat hit the deck when hearing thunder?

A soldier who had spent time in combat is a really a very tough time, where a soldier is required to stay attentive.  

Explanation:

The soldier must be very cautious each and every minute, because at any moment the opponent may attack him. So he remains active after the war also. A soldier is a brave heart who serves for his country, he doesn't care about his own life when the matter comes to the safety of his country. He is always ready to give his best efforts for his country. Therefore, it is said that a soldier who spent time in combat hit the deck when hearing thunder.

answer normal force;