Final answer:
The collapse of the Indus Valley civilization is thought to be caused by environmental degradation, climate change, and disease. These factors weakened the civilization and made them vulnerable to attacks.
Step-by-step explanation:
The leading theory about the collapse of the Indus Valley civilization involves several factors, including environmental degradation, climate change, and disease. Exploitation of resources and high population density led to environmental issues that could have been worsened by climate change and disease. The weakened state of the civilization made them more vulnerable to attacks by raiding groups like the Aryans.