Its literally in the rules -
"After some time it will be checked if the leading tribe has reached a certain amount of influence points. If this isn't the case, the required amount of points will be slightly reduced on a weekly base. The required points won't be reduced anymore if the leading tribe reached a certain threshold."
Great siege points dropped by 12% per week, HS reached 80% of the required points so the drop stopped. The problem was that they had no more villages to improve their score, so Op7 overtook them. There's no point in waiting another week to make 22k points so administration ended the world.