In Iraq, Shiite Arabs are the majority, and they control the government. Then there are Sunni Kurds who live mostly by themselves in northern Iraq. And then there are Sunni Arabs who live in the western half of Iraq and are a political minority. Many Sunni Arabs are the ones joining and supporting the ISIS takeover.
Many Sunni Muslims feel they were mistreated under the Shiite prime minister. Since Shiite Muslims are the majority, they tend to elect Shiite leaders, so Sunni leaders may never be in top positions of power. Because ISIS is Sunni, many Sunnis support them and see them as liberators.
Not all residents are supporting ISIS. Many who fear ISIS and combat have been forced to flee Mosul and Tikrit as ISIS has taken over.