Inertia? Safety?
Maybe inertia overcomes air resistance. Or maybe it takes another 39 seconds to reach a safe splashdown location for the boosters. Or maybe they want to carry them higher so they'll burn up instead of splash down. Or maybe they want to expend more fuel, to make the second stage lighter before booster separation - that way it'll accelerate away faster with less chance of a collision.
Or maybe it's a typo.