Good post. I had noticed some of the same, although it didn't click about the draw until I saw what you wrote.

The AI is aggressive when you try to leave your fortification since you are vulnerable when you sally out. I've had them rush in to batter a unit or two, then pull back to a respectful range.

Alas, the AI in general will let you use slingers to cut them to ribbons if they lack the force needed for a headlong charge.