It's unlikely they became nomads. They probably just moved to a different plain and continued their agricultural ways. I am already boring you and I haven't even told you yet how to be a better boy and or girlfriend. I'm going to do that now. So we don't know why the Indus Valley civilization ended, but we also don't really know why it started. Why did these people build cities and dig swimming pools and make unnecessarily ornate seals? Were they motivated by hunger, fear, a desire for companionship, the need to be near their sacred spaces, or a general feeling that city life was just more awesome than foraging?