The AI is trying to spread out the units to cover the front line equally. Here's the problem: in order to have a planning bonus, you have to let the AI control your units. If you give manual orders to units, they lose the planning bonus three times faster than if they're under AI orders.
You can finagle the AI into doing what you want. For example, you can put your breakthrough units in an army with a small frontline that borders the province(s) you want to breakthrough, and your defensive units in an army with a longer frontline that includes it. Give the breakthrough army an offensive line or spearhead, and the defense army will move units to cover the territory they take as the frontline moves.