INCOMING THEORY! BRACE FOR IMPACT.
The Imperial fleet stationed around the DS2 had orders to hold their position. Only the Admirals likely had any clue as to why, and even they probably didn't know precisely what was planned.
"Should we attack?" "I have my orders from the Emperor himself. He has something special planned for them. We only need to keep them from escaping."
When the DS2 blasted the first Rebel ship, the Imps all likely did something like this: "So that's what he meant. Awesome. Time to put our feet up and enjoy the fireworks, maybe pick off a few stragglers."
Then the Rebels charged headlong into the Imperial fleet. I imagine it was similar to Han charging a squad of stormtroopers in ANH. They figured that the Rebels/Han obviously had some sort of secret plan, weapon, or backup that they were hiding, and subsequently panicked and/or decided to play it defensively.
Then you have disorganization as the lower commanders deploy forces to react to what's happenning while the higher-ups try to figure out what's happenning and why the plan isn't working. Nobody coordinates, the Rebels press their advantage using crafty tactics, superior fighters, and movie magic.
But still, the Imperials eventually start to turn things around. "Come, boy, see for yourself. From here, you will witness the final destruction of the Alliance and the end of your insignificant rebellion." But it's too late; soon the Emperor gets the shaft, the DS2 gets it's reactor shot, and the Empire's most powerful person and weapon go down in a matter of minutes. Obviously the Rebels did have some sort of fancy weapon or plan, and they're using it! Run away! And the Zahn theory thing too.