The  possible reasons for the decline of Indus civilisation are flood, migration, invasion

The decline of the Indus Valley Civilization around 1900-1700 BCE was likely caused by several factors: environmental changes, including climate shifts and river shifts that affected agriculture and trade routes; internal factors such as overuse of land and resources leading to soil exhaustion; and external pressures such as invasions or migrations of Indo-Aryan peoples. These combined factors weakened cities, disrupted trade networks, and contributed to the eventual abandonment of urban centers, marking the end of the civilization’s urban phase.