From Season 4 Episode 10 Faq: All Major Plot Questions and Ending Answers Solved
The finale took its title quite literally. For three seasons, the fallen tree across the road served as the symbolic boundary point, the entry snare that trapped every motorist inside the township's impossible topography. In Episode 10, that physical anchor shifted. When Jim and Kenny ignited the deadwood barrier deep in the northern woods to signal the radio outpost, the resulting fire triggered an environmental feedback loop.
The town is not an open piece of land. It operates as a closed bio-circuit. Destroying the peripheral timber did not clear a path back to highway networks; it caused the township's borders to contract violently. Houses on the outskirts, including the remnants of Colony House, began shifting several hundred yards inward over three hours. This contraction forced survivors into a dense cluster in the center square, rendering their protective spacing obsolete.
The physical compressions triggered secondary tremors throughout the cave network. As the soil broke open beneath the storage sheds, the runic stones hanging at entry points lost their energetic balance. The stones did not fail because the symbols faded. They failed because the ground beneath the doorframes ceased to belong to the dimensional layer the talismans were designed to ward.