The muscular guy does something which surprises him.
Here’s the thing about all levels of combat that most people seem to miss. They think, “oh, I have a certain level of skill and therefore I’m safe”. Or in this case, “my character has a certain level of skill, they’re unbeatable by anyone less skilled than they are”. The less training someone has, the less they’ve internalized how little they actually know, the more likely they are to be surprised and ultimately undone by an attack or behavior which comes out of left field.
There’s a certain level to fighting which is about expectation, you can only prepare for what you know and, as with anything, there’s always the risk of growing too comfortable. Overconfident. The character thinks, “hey I can do this, I’ve survived this, I’m tough shit!” Which you know what, maybe, but it’s when they start looking down their nose at someone they believe is less than they are that they get clocked. Skill is not actually as represented in a video game. It’s more like Rock, Paper, Scissors except that every so often someone breaks the rules and throws down Bomb, or Glue, or Lightsaber or one of the other really weird invented ones which don’t belong.
They changed the rules, laugh, and say “haha, I won”.
The irony is that people with no fighting experience do have the opportunity to change the rules on someone with some experience, depending on that level of experience. The reason being that you come into a fight with a certain expectation for how they’re supposed to behave. When we’re talking about bullies, this is how bullies operate. The bully operates from a position of power on an understanding of behavior within the social strata both on how their victim will operate or react (how likely the bully is to get in trouble) and the likelihood of reactions on the parts of authority figures. The vast majority of bullies strike from a protected position of power.
Your character went into their altercation with an expectation of certain behaviors from the other guy, the other guy responded in a way that they weren’t prepared for, and they get hurt.
The problem with the way training and any kind of combat experience gets treated in the media is that, for the most part, it refuses to accept human limitations. It’s the basic problem where when one doesn’t understand how something works they go, “magic!” Also, video games where there are a strict set of unbreakable rules. Like taking a Level 25 character into a Level 11 zone and they roflstomp their enemies with one hand tied behind their back.
There are no guarantees of anything working. Ever.
Like the varying levels of martial training, experience with fights means a potential greater awareness of surrounding and a slightly higher ability to predict what someone will do. Being able to predict what someone will do gives a better chance of noticing when they’re going to do something and react in time. Whether it’s in seeing the beginnings of movements which look familiar, remembering that even when the other guy is bleeding on the ground he’s not necessarily subdued, or what have you.
Your character made some kind of mistake or miscalculation. That miscalculation lead to him losing control of the fight. When he lost control of the fight, lost the initiative, had the tide turn against him, he wasn’t able to right himself (which is pretty damn hard to be honest).
Every single person, no matter their size, their height, their weight, their gender, or their level of experience, has it in them to hurt someone else if they get pushed to the point where the fight instinct overcomes their flight instinct. They may not be very good at it, but that doesn’t mean they won’t be successful.
Try to remember that your character is limited by the virtue of his experiences. He is not omniscient. Don’t let him peek over your shoulder. Keep the dungeon master screen up at all times.
And think about it.
Where is his head at? What doesn’t he expect? What would surprise him?
And for the other guy.
What is he feeling? Pain? Guilt? Humiliation? Fear? Anger? Pure undiluted rage? What is going on inside him that could lead him to just snapping and going for it? Is he the kind of guy that’s just going to lunge, tackle the other guy, go down on top of him, and just start pounding away at his head with wild swings while screaming at the top of his lungs?
It could be anything.
The less training someone has, the more instinctual and gut reactive their response. And that means when pushed to a certain breaking point, people can get a little… base, I suppose. That can get terrifying.