The Physics Acting on the Board

Even if you try hard to lift your front foot, the board can sometimes rise faster than your foot. When this happens, your front foot becomes relatively slower than the board, causing it to apply a downward force on the nose. You might think, "Pressing the nose down shouldn't affect the board's center of mass," but from a physics standpoint, no matter where the force is applied, any downward force on an object equally opposes its upward motion. In other words, you're not intentionally pushing the nose down, but if your front foot is too low or moves too slowly, the board will unknowingly lose some of the force that would otherwise lift it upward.