This usually occurs when you overmix your cupcake batter (see tip #4 above!) and/or when you use ingredients straight from the fridge instead of letting them come to room temperature first (like eggs, milk, or butter). If you'd like less tunneling in your cupcakes, make sure to bring your eggs and milk to room temperature before making the batter (see tip #2 above), and mix your batter until everything is just combined.