At least in the alpha, the alliance were there specifically to launch an attack of opportunity on the forsaken fleet because they learned Sylvanas would be there. Genn even says he doesn't track something unless he intends to kill it. The forsaken were there to get the pillar. The alliance were there for revenge (because they're not smart enough to ask Sylvanas why they retreated from broken shore apparently).That's pretty how it goes Alliance side, one minute you're just doing preperations, the next, fight goes down. Really there's no single blame. It's just the narrative trying to tell you "SEE THIS SIDE IS ATTACKING YOU!"