There was this girl. First mutant born since M-Day. Cable took her into the future to raise her. When she came back it was this whole big thing. Purifiers, all the people that really,really hate us went full out war on us, trying to get her before she could make it to Utopia.
Then after all of that, the Phoenix came calling. Avengers decided that they knew the Phoenix better than we did, even though we have people who hosted the Phoenix for years without problems.
Stark made a device to stop the Phoenix from bonding with the girl. Phoenix splits into five of us. They just wanted to do good with the power, but you know how power corrupts. Between that and everyone constantly attacking them it was only a matter of time until someone snapped.