Knock out is caused by a chemical released when the brain receives a sudden shock, bangs against the skull or when the spine seperates slightly from the brain. This is an involutary action caused by hormone release in similar way to the hormones stimulate a boner on sight of Arianny's playboy spread). Its the bodys way of shutting down to protect vital functions like respiration. The effects of the hormone can last a while. My mum had a small stroke was told that she could be slowed down and tired for 6-9 months. This proved to be accurate.
When a fighter suffers a bad ko he is more inclined to be victim again. this is due to more readily produced hormone.
Aged fighters tend to suffer kos as their muscular strength in the neck can weaken as well as their reactions slowing down. Weaker muscles can make it harder to prevent a whiplash effect with a good chin shot. All this adds up to weak chin. Rashad KO'd Chuck with a blow that crushed chucks neck vertibrea in a downward motion and you can see them stick out from the reverse angle
I guess it's not the amount of beatings but the sweatness of the KO shot combined with the slow down in reaction time of the fighter and weaker neck muscles.