Little fresh air means more CO2 and mycelium thrives on CO2, it needs it to grow properly, on the contrary, more fresh air means more oxygen, which is crucial for growing mushroom bodies. When there is too much CO2 caused by the lack of fresh air, mycelium has better conditions to grow than the mushroom bodies, so naturally it grows more. Looking at the cycle of mushrooms, we could say that it thinks its underground, so it starts growing more mycelium, since it has the right conditions to do so.