One potential cause of the decline of the Indus River Valley Civilization is climate change leading to altered monsoon patterns.
Firstly, decreased rainfall could have resulted in water scarcity, affecting agriculture and food production.
Secondly, changes in the river's flow may have led to flooding in some areas and droughts in others, disrupting trade and daily life.
Lastly, the resultant social stress from resource shortages could have caused internal conflict and contributed to the civilization's eventual decline.