Mosul is a key city because it’s a key point of friction between the Kurds and the Sunnis. Juburi’s appointment was not without controversy and violence, to say the least; his early speeches resulted in a demonstration during which US troops fired on demonstraters. The details of the event are unclear: the US claimed that the demonstrations were violent and the demonstraters claimed they weren’t, which is as you’d expect in any case for both sides of the story.
This means that Mosul’s had two governors leave office in the last few months, by the by. Not exactly a model of stability.