The Kirani Sentinels attacked as soon as Adam and Fromund entered the inner territory of the colony.
Despite their size, unlike the Kirani Guards, they were the Awakened Monsters, although they had difficulty speaking. They preferred to remain in their monster form to gain maximum strength and be able to use their shields.
If they were fighting for victory, they would not have done so, because when the forces were roughly equal, being bigger was a disadvantage. A smaller enemy had more room to maneuver and was also faster.
However, the Kirani Sentinels did not want to defeat Adam and Fromund; they gave up on this goal because they understood that it was hardly possible.
They had to do what was best for the colony. In this case, they simply had to delay Adam and Fromund as long as possible.
The Kirani Sentinels and thousands of other bugs believed that once Bathild and Marhen arrived with the thunder monsters, it would be all over for Adam and Fromund.