So something about that place, probably similar to the 'Chigra' effect, causes some kind of visceral and instinctual reaction, as well as heritable to future generations. The men that emerged carried within them a piece of that place, maybe due to the fact that it was a topos and their being was fundementally changed, and that piece triggers whatever 'flight' response mechanism that we see the sranc exhibit.
Might have been useful to force a few more legions to spend the night there. Could have had them a bare handful stand guard over the Schoolmen on their mountains, guaranteeing their safety a bit more.