Most news reports bring the fall of Mosul to ISIS as something sudden and unexpected. My interest was triggered by a (Dutch language) report from some people with local connections. The report paints a landscape of were most people are against the government, the army is seen as an occupational army and the police isn't very popular either. The sectarian politics of Maliki, widespread unemployment and the army attack on the protest camp in Hawija have led to a broad coalition where local tribes, the (underground) Baath Party and ISIS all work together against the government.
The big question is whether this is only local. The protest camp in Hawija showed the kind of color revolution tactics that suggest foreign involvement. It is well known that the Saudi's would love to turn Iraq back into a Sunni dictatorship. It is also well known that ISIS is well funded and rumored to be supported from the Gulf States. So I suspect that the Saudi secret service has a hand in what is now happening in Mosul. And very likely at this very moment Saudi diplomats are active in Washington to prevent US support for the Iraqi government in its fight against ISIS.