Board Thread:Episode Discussion/@comment-410526-20160116155944/@comment-26872876-20160117212213

From world of Remnant we know that Grimm are drawn towards fear. It isn't so much they knew it was time as it is enough of them appeared and the fear was high enough for them to move in. The Grimm we've seen have behaved rather animalistic. Elephant like Goliaths move in herds. Beowolves have packs. We also know with each battle, a Grimm becomes more intelligent and experienced.

The only hive mind they seem to share is killing humans.