It is, but it's Blizzard writing so just go with it. It was a very focused stab. But if the stab, or N'Zoth rising or anything like that causes a mini cataclysm in Lordaeron (a region not affected that much by the actual cataclysm), that would explain why the Alliance forces attack. The might figure that with the region being vulnerable it would be easier to take Sylvanas. They figured wrong, but still it is not a bad logic.