I think the AI sometimes gets into a situation where two or more courses of action completely balance each other and as soon as it moves one way or another, the balance tips against that course of action and it tries another - or none. Then, when it can't see a good move especially if it's taken a few losses, it withdraws.

I've seen the column thing before, but usually in seiges. The AI is on attack but can't see how to proceed so lines its troops up waiting.