So you saw the difficulty change on screen, while there were no reinforcements nearby? I haven’t seen that before. Could be a bug. You should see if it recurs and post the log in the bug section if it does.
I’ll keep watch for it. My guess is that it could be caused by pending quest for eradication of this hideout, so it refills the population as I come to it. Anyway, looks strange and a bit wrong.
