Why did E L S N Bala Prasad miss the bus ?

Trusted IAS officer of the President Ram Nath Kovind, E L S N Bala Prasad missed the bus of his posting as Secretary and K D Tripathi was brought in as new Secretary .In fact, Bala Prasad was Principal Secretary to the President Ram Nath Kovind when he was Governor of Bihar. 1986 batch Bihar cadre IAS officer was recently empanelled  as Secretary and it was almost certain that he would succeed Sanjay Kothari as Secretary to the President. What is the mystery ?