I'm not too sure.. had my first siege yesterday (I've been playing to and from, mostly building and designing instead of waging war)
The enemies were a group of trolls, a group of 16 mounted pikegoblins and 16 mounted bow-goblins.
The trolls killed a chained cat, I raised my drawbridge and they figured they were done for the day. The trolls left.
The goblins killed chained animals as well, I lowered the bridge to lured the bow-goblins near and raised it again. Dwarves began firing from the battlements ... and they didn't hit a single goblin, they only hit the mounts. After 3 mounts had passed out and 5 were somewhat wounded, the goblins began retreating. The leader lingered around for a bit but then the entire squad ran.
So with only one group left I lowered the drawbridge and prepared my dwarves for a battle. The leader didn't advance, instead 7 of his troops broke off from the group and came rushing in. They fell in a few traps and were shot at, the mounts died pretty quickly and the goblins were thrown off. The siege was broken and I figured I had won! I sent the melee-dwarves out and they executed wounded mounts and goblins on the bridge, and indeed the enemy HAD retreated ... all except the leader. The goblin hammerlord "Utis Evildeer" who lead the pikemen.
He charged and killed two of my best warriors, crushed the hand of a footman and broke the legs of three footmen. With only two dwarves present and standing, the only thing that hit Utis was two bolts that barely grazed him. He finally fled and left me to collect my wounded and wonder what on earth happened.
I imagine that Utis flipped my dwarves off before running into the desert.
TL;DR: In my siege the goblins acted independent of the leader, but I interrupted their path-finding with a drawbridge more than once. IDK if that has an effect.
ps. Utis wasn't technically the last goblin. There was a pikeman mounted on a jabberer being chased around the entire map by a scrawny little dog.