The people during the Zhou Dynasty perceived natural disasters such as floods or famine to be indicative of their rulers not following the Mandate of Heaven.
The Mandate of Heaven was created by the Zhou and means that only one person who is believed to have the blessing of the Gods can be the ruler of China, and there must be only one ruler at a time. So, every time a natural disasters occurs, it is interpreted as a sign of heaven's disagreement or lack of approval of the current ruler according to he Mandate of Heaven.