What Happened in the Indis Valley?

There's very little sign of outside attack on the indis valley cities, it seems there was much more internal breakdown. The indo arian people didn't arrive in this region until about 15 hundred b c. So there must be some local or internal explanation for what went wrong in the indis valley. And to day, it's more widely thought that it was due to droughts and climate change. It might also have then been exasperated by earthquakes. More broadly speaking,the decline of this civilization was part of a wider late bronze age collapse.