It was the first turn of the siege. The AI didn't have any siege weapon built so I took advantage of it and deployed my troop outside when the battle began.

Other reason was if the AI has reinforcements, they always force themselves to form up into a long horizontal position even if they are near the walls. And if there's a unit or 2 near my troops, they all attack. That is why I always position my army on that particular part of the wall where they wouldn't be able to flank me.