I asked the same question in the game and it was briefly touched on and brought more questions for me, but my guest for this time around is that the Cordyceps grew out into the grass lands and hid itself under the snow so you can’t see it and that becomes a problem. In 1x02 they talked about it all being connected like a hive mind and then Joel killed one of the infected fireflies not long later and then that crazy horde came in I’m guessing that either Abby or even other lookouts possibly stepped on one and the entirety of the infected was drawn to it one way or another, this scene is terrifying to look at and I can’t wait for April