I don’t believe for a second that Assad forces did this..especially while the UN team is there looking into WMD use in Syria. Either AQ forces did it to gain sympathy or some Assad shelling hit a WMD stockpile in rebel hands and caused the release.
Was thinking along the line that it was a random hit of material secreted away. They may have targeted a known ammo stockpile without knowing the type of ammo IN the stockpile