You only get to stand in the aisle to get down a bag if there is no one waiting behind you. The theory is that, front to back, aisle seats take down their bags before the gate opens and they all walk straight off the plane. Then middle seaters all take down their bags at the same time, and all walk off quickly with no pausing. Then the window people get to deboard.
But we are all too selfish and impatient for that to work.