Major causes for the decline of the Indus civilization are,
The conquest and destruction of their important cities by the powerful set of invaders (probably Aryans) declined the civilization.
Natural calamities such as wildfires, floods or severe earthquakes might have resulted in the damage of the cities and ruined them. For example, Mohenjodaro was rebuilt more than 7 times.
Shifting of the monsoons and soil erosion made the people to desert the places. For want of fertile land, the people might have abandoned these places.
Spread of epidemics must have wiped out the population.
The rivers might have changed their course and ruined the cities. The drifting away of the rivers from the cities might have rendered the place unfertile.
