Trump had in previous weeks expressed admiration for Bharara and wanted him to stay on. But this abrupt 180 turnabout comes as opponents of Trump in the US Congress have called for Bharara to lead an investigation into the Trump Whitehouse over ties to Russia as well as Trump's financial dealings.
Trump may be pre-emptively moving to eliminate Bharara before he can be turned into thorn in the side of the Trump administration.
UPDATE: Bharara refused to resign, effectively forcing Trump to fire him
I did not resign. Moments ago I was fired. Being the US Attorney in SDNY will forever be the greatest honor of my professional life.— Preet Bharara (@PreetBharara) March 11, 2017