The monster and the technique he released were both entrapped inside that shield. And he'd either had to wait for the technique to get exhausted, or the monster to break the shield and escape that hell. Either way, William was sure the bear monster wasn't going unscathed from this.
The monsters were trying to reach the underground tunnels and caves at first. But when they all realised there wasn't any threat, they stopped and even returned to surround the hole and the dome covering it, as if they were waiting for something magical and big to happen.
The moment William shouted, they all knew something bad was going to happen. They couldn't hear anything, but they could see the cracks spreading over the shield with every violent shaking.
Without the need to ask, they all knew this was the cause of the monster entrapped inside. And it was better to run away into a safe place and wait, rather than face the wrath of that monster or get hit by the entrapped technique of William.
