After taking down the first two major bases, Sera and Leo continued from there onward. Both of them were surprised that the other had succeeded and actually believed that perhaps the other would fail. As a contingency, the plan in their head was to move to the base where the other had just lost, or to help them out.
Yet, they had both been successful and had managed to finish the task quite quickly. Because of this, they decided to continue attacking other major locations. Since the third one was on the flight path of both of them, they had decided to attack it together, which had only sped up their pursuit.
The Shelters were expecting their defences to last weeks and for attacks to have weeks in between. But in truth, they were falling in a matter of days with little rest on both sides.