My guess is it's because of your use of "trigger once" conditions.
As I see it, when the first blue minion gets to the first tower, he triggers the "On pathfinding arrived, bluepathfinder=1" condition, and increments his own bluepathfinder variable to 2.
He then matches the following condition, "Bluepathfinder=2, trigger once", and sets about finding a path to the second tower.
However, when any subsequent blue minions arrive at the first tower, they increment their own bluepathfinder variable to 2, but never fire the second action to plot a path to the second tower, because it's already been triggered once (by the first minion to get there).