I know nothing about martial arts and I'm not sure this is the right board, but I recently read about the Conor McGregor vs Jose Aldo fight which lasted only 13 seconds. How did Jose Aldo lose so fast? Is this a typical fight time?
Conor McGregor punched Jose Aldo in the head. The punch was quite hard and accurate. Aldo got hit while out of position. Aldo didn't see the punch coming. All of these factors, plus a dozen other intangible ones we can't quantify, lead to Jose Aldo getting knocked out.
When someone gets knocked out, they often can't just get up again. In particular, while Aldo was trying to regain consciousness, McGregor was continuing to hit him in the head, which tends to re-concuss people who are already concussed.
The referee saw that Aldo was unconscious and therefore not defending himself. That's sufficient reason to end the fight, which he did.
That's how Jose Aldo lost.
The really simple answer is that he walked into a punch.
The more complex part is that this is effectively what boxing, and many other striking arts build their entire science around - "How do I get that shot in that they're not ready for?"
The flipside, is that the striking arts also spend a lot of time training to take hits - to angle the blow off a bit by using your shoulder to tuck your chin, to give with the hit and take off some of the force, and definitely not take the shot full on. If you're not ready... well. Here's a half hour compilation of boxing knock outs and you can see a lot of similar situations.
Counter punching has the advantage of being sneaky - the counter attack is often masked by the attacker's focus being on their target, not the incoming attack, and their own arm or shoulder might be blocking their sight to even seeing it coming.
It has the second advantage that you have more force - if the attacker is moving in with their body weight and you hit, you have both your force and their force added together (much like how head-on collisions between cars is an additive force). A concussion isn't surprising really when you consider all of that.
Aldo was coming in and throwing punches and caught the counter. He walked into it.
Obviously, this isn't the normal fight time, otherwise we'd be seeing 50 card matches every fight night. For a good while, Mike Tyson was making his name on quick knockouts as well - against professional heavyweight fighters. Most of the time, however, people spend a lot of time beating each other's endurance down in order to find that opening that will let them land the killer shot, and, often, in striking sports, they win by decision rather than knockout.
A simple answer is that he was punched in an unprepared moment. Here it happens while the fighter rushes forward attacking and receives a counter blow at the right spot to the head.
This blow to the head is causing a shake of the head in that way that the brain hits the skull what causes the nerveuos system to shut down and it results is the knockout.
Usually fights don´t end so quick, it really depends on the skills of the fighters, preferences of fighting style and sometimes even luck is involved. Just a side note, in a fight causing that kind of knockout is not really easy, if tried on purpose.