Just as Erin declared those words, she heard the sounds of leaves rustling and twigs, indicating that something was moving towards her from within the forest.
Erin had purposefully gone a bit further away from the shelter so that she could vent out her anger in peace and have no one disturbing her. She had a general sense of the time that was remaining before the next wave arrives, but it turned out that she was so focused on stabbing that tree that she completely forgot about the time.
Additionally, there was also the question of where the 4th wave would come from. The first wave of Silver Wolves had attacked from the west, from directly behind the shelter, which was why they had caught them by surprise. The second wave came from the south while the third wave came from the north. This left only the east side and Erin had a hunch that this is where the beasts would attack from this time, and she was right.
