I think Al Qaeda’s leaders realize that ISIL atrocities are too extreme even for very conservative Muslims. Al Qaeda knows that ISIL is so bad Islam is loosing support even within Islamists who want Sharia Law.
ISIS has been attacking Al Qaeda ever since it crawled out from under a rock. The Yarmouk Brigade of ISIS that was attacked had taken over areas adjacent to Israel, and said they had no problem with Israel (obviously that's an exigency); meanwhile Assad enabled or collaborated in ISIS attacks against Al Qaeda, and left Hizbollah to fight ISIS which was only a priority because ISIS was trying to take over the areas on both sides of the Syrian border with Lebanon. It was getting so bad that the mass-murdering nutjob the runs the Hizzies said they'd prevail against ISIS if half of Hizbollah had to be martyred.