I've never seen anything comfirming or denying it but everything runs towards bastions because while it's supposed to be one 90,000 hp (for example) piece the attack 'logic' (haha) considers the part of it it wants to break as 90,000 ÷ 6 = 15,000 hp piece. In other words, far, far, lower than any of the walls or even gates it's attached to. Since the AI looks for the path of least resistance they zero in on bastions then spend precious seconds idiotically dancing around in the rubble.
Really obnoxious, but sadly, par for the course with this dev team. Hell, from what I can tell they've never even fixed the bastion placement bug in the two(?) years since revealing them. And of course, I'm sure the plan is to allow people to build two more bastions with each subsequent age until the game is rendered completely unplayable.
A little off-topic, but one of my pet peeves when I log in every few weeks.