X-Men: First Class overcame many hurdles from losing its original director Bryan Singer (a coupe getting him to return) and his late-call replacement by Matthew Vaughn (himself Singer's original replacement on X-Men: The Last Stand before he too quit) to meeting its nut-crunching June 2011 release date (Filming started in Fall 2010) to it being good (A rarity in the sub-genre of prequels).
Then it under-performed and the prospects of future installments with James McAvoy, Michael Fassbender and Jennifer Lawrence went dim. Probably due to their desire to keep the property on their turf, 20th Century Fox set forth on a sequel, signed down Vaughn to return, a July 18, 2014 release date and the promise of adapting Days of Future Past, a not-subtle way of saying this was their plan to reboot.
Everything's a-OK now, right? Nope, here's a curve-ball thrown at the sequel, via "TOLDJA." Vaughn's bailed. No reason cited (He apparently will stick around with Fox) and since they have that July '14 release and a start of production geared for early next year, they need a replacement. And the rumor-mill says Singer (who served as producer on First Class and from what we’ve heard played a much bigger hand than let on) could step up to direct.
All due respect to Singer, who I continue to be a big fan/defender of, such a last minute move by Vaughn can't be good. Something's up.