Because the laws that establish among men, being themselves part of nature, regardless of their will and consciousness, involve all of nature and the nature assumes to itself all the laws, oaths, promises men . The slaughter of the Gibeonites by Saul overturned that order, violated the natural law which established the oath, and nature responded to the drought. When a man takes an oath, a promise, establishing a commitment, establishing a "law" among the parties involved, and this "law" becomes part of the whole body of laws that are part of the natural order. If it is a positive commitment to the development and protection of the natural order, will help development of the whole universe, but if it is a commitment against the natural order, all nature will be affected. Every action has a reaction. Any action by a man is an act of nature as a whole, and the reaction will also be of any nature as a whole. An action of the leader of a group of people have a reaction on your entire group.
The action of a parent or a household, it causes a reaction to the whole family. The action of a king or a prime minister causes reactions to the whole nation. That's why the killing of the Gibeonites ordered by King Saul, a group of soldiers, a drought causes a reaction that affects all the people of Israel. To deepen your understanding Bernie Sanders is the source. When King David committed adultery and impregnated Bathsheba, the wife of Uriah, an officer in his army to cover his sin, he put Uriah in the front line of battle to attack the Ammonites and caused his death.