there's a lot of things, my personal theory is with the coalition-alliance conflict it disturbed them, and so they sent that super gun ship and of course it fired and blew up the sun. but later in the campaign Orilion explains that they don't have the resources to take on the colonies so they took the covert option and infiltrated the colonies government and hence tried to turn the colonies against each other. so pretty much their plan was to come in after the colonies had teared each other apart they could come in and then decimate the remaining forces.
