How did the Harrapan Civilization fall ???

The Harrapan Civilization fell because of D.I.R.E. and P.S.

Now what are D.I.R.E. ( trouble ) and P.S. (post scribe )..
Lets see................

D = Natural Disaster
I = Invasion of Aryans
R = Change of the course of River Indus
E = Economic problem
and
P = Over Population
S = Salinization of Soil