Is the US is after changing the leadership in Pakistan? If so, WHY?

Imran khan has named a top US official who was allegedly behind the 'threat memo' seeking his ouster as PM. Pakistan premier claimed that Donald Lu, the US Assistant Secretary of State for South and Central Asian Affairs, as the official who had allegedly in a meeting with Pakistan ambassador Asad Majeed warned that there could be implications if Imran is not ousted.

The question: Why is the US is after changing the leadership in Pakistan?