Any number of reasons - spring housekeeping, he ticked someone off, wasn't focusing on this administration's priorities, etc? Doesn't matter.
Cool with me, Why would anyone question it?
Firing a U.S. Attorney does not make the case go away. The U.S. Attorney is the person in charge, the government employee lackys who do the work are still in place. It is still the case that they can be fired for ANY reason including political.