Stress and blood pressure builds over the course of the fight, and the volatility of the boxer increases as he gets tired and damage is done.
By the end of the fight, it takes much less of a blow for the boxer the cross the pressure threshold and lose consciousness. So even as the punches lose power, the boxers are still more and more likely to get knocked out.
Each punch raises the internal pressure of the brain system of the boxer. While they recover between rounds, and between blows, the threshold for a mini-stroke becomes lower and lower. Once that threshold is reached, lights-out.