Sadly, the most accurate answer is "we just don't know." Wiped out isn't the correct wording however, think more "changed and moved on and away." Every civilization falls.
There are many who attribute the decline of the IVC to climate change (see the wiki link below) and this wouldn't have been catastrophic. Make sure you read the part of the wiki article entitled "Late Harappan" since that has what you are looking for.
If you are looking for references, I would suggest any of Bridget or Raymond Allchin's books.
