There are many distinct theories about the origin of Indus Valley Civilization but not one of them has been completely confirmed including the so called Dravidian theory. The scholars mostly agree that Indus Civilization developed from agricultural society that used the Mesopotamian version of irrigation.
Like source, meltdown of the Indus Valley Civilization remains unknown but it was certainly not caused by a single factor. Mohenjo-daro was devastated by floods for many times and it appears that the city has been in decline already for some time before it eventually ceased to exist. The decline of Indus Valley Civilization may also be due to climate fluctuations, exhausted soil, a disease and invasions by foreign populations. There’s been a lot of speculations whether the Indo-Aryans were accountable for the final blow to the Indus Valley Civilization. Archaeological excavations in Mohenjo-daro revealed unburied human remains on the streets of the city but there’s not any firm proof that walled cities and citadels that were according to the novels of Rigveda assaulted by the Indo-Aryans were actually referring to the cities of the Indus Valley Civilization. Even more, the archaeological evidences suggest that Indus Valley Civilization collapsed before the arrival of this Indo-Aryans, although the remote cities south west from Gudjarat seem to survive the collapse of both biggest cities of the Indus Valley Civilization.