The enemy hadn't shown themselves from the beginning until that massive Dead Star Fortress entered the final blockade zone of the Big Nebula, even after a superweapon had blasted a two light-year-long hole into it, they remained motionless. Now, they all came pouring out at once.
I didn't know if this was a tactical choice they made on purpose. If so, it could only be said that, after several failures in the past two days, the Rebel Army had smartened up a bit.