Why did Iyer resign?

Why did the face of Swachh Bharat Mission, Parameswaran Iyer Resign ? Drinking Water and Sanitation Secretary Parameswaran Iyer considered as blue-eyed boy of the PM Narendra Modi has  stepped down from his post and his resignation has been  accepted with effect from August 21. Uttar Pradesh cadre 1981-batch IAS officer (retired) had been given one-year extension till 30 April 2021.It is heard that he is returning to Washington where his family resides. Meanwhile, Power galleries were on Monday curious to find out the reason why Iyer resigned from the post of Secretary ? Many rumours were also floating but nobody was sure about the reason behind his sudden resignation.