Pretty straightforward, this is a fort that has a siege of ~100 goblins, and as soon as each of the squad leaders dies, the rest of the ~90 or so goblins stop pathing/invading and stand around doing nothing.

As mentioned previously, a quick and easy solution to this problem is that if a squad leader dies, a squad member is instantly promoted to leader and continues the invasion, repeating as necessary until there are no more squad members, or it's time for the squad/invasion/siege to flee.