For me, two situations prevail: either it attacks a breach or it becomes passive after it runs out of viable options.
Quote Originally Posted by dismal
Yes, I am inclined to agree. The AI attacks robustly enough when it thinks it can win. When it doesn't it'll just stand around. Unfortunately, this is not consistent with the defender's advantage I work rather hard to earn.

I have experienced the problem not only in sieges but with hills. I get a nice defensive position on a hill and the AI won't come up and play with me.
Yep, I think this the best explanation.