For example, guys like Stephan Struve and Arlovski. All their kos are highlight worthy. Arlovski looks like he got shot when hes gets knocked out. Is it because of bad chins?
Think of it as if Optimus Prime was designed out of titanium but they used a teacup to make his chin.....
ttt
Arlovski drops his hands alot.
Struve has not caught up with his developing physique...
and apparently leaves his hands down/chin up while throwing a flying knee.
Struve's striking defense is bad as well.
Sofa King Cool
7 hours ago
Chuck also drops like a rock.
Wasn't always that way but yeah.... Chins can only take so much.... Once it's gone it doesn't come back 
From his very first fight against Datsik it's been evident that Arlovski's brain just can't take getting rattled, it shuts down.
Every human brain is different and reacts to trauma differently. Neurologists will tell you how different and unpredictable trauma dynamics are.
Also once you've been concussed you are more likely to see it happen in the future. And part of it is genetic.
Look at the Lindros brothers. Both were highly concussion prone. Partly due to playing style, but plenty of guys have that style without having "glass jaws". But both brothers got concussed - and after that they got concussed pretty easily. They were prone to concussion by genetics, and once it happened it kept happening until it ended their careers.
Happens to NFL QBs too, once they get concussed badly they tend to have recurring issues.
Yes some of it is defense, but Arlovksi does indeed have an iffy chin. He ate some shots from Bigfoot but that doesn't mean his chin is solid because it's very random. Punching the head is very chaotic so you won't always get KOed - sometimes you'll eat a hard shot fine, but a weak shot finishes you.
Old Chuck had one of the worst chins ever. But the glass jaw GOAT has to be Jorge Santiago. Just so many embarassing KO losses for him - KOed by hammerfirst from bottom, KOed by a featherweight, murdered in all his UFC fights.
Go get punched and see how your body reacts!
We're all different!
The reason Struve gets KOed is different than Arlovski though, Struve's chin might be iffy but his problem is definitely more defense. Struve has zero defense. He just sticks his chin out and throws punches without putting his hands in any position to block.
He also moves very awkwardly and leaves himself wide open continually. Struve has shown no improvement in this area either. I have no idea what is wrong with his coaches that they haven't addressed this.
Arlovski and Chuck also were aggressive and left themselves open, but it doesn't compare to Struve. Chuck's aggression worked for him until his chin went away, and while Andrei isn't the best defensive fighter he isn't horrible - he just tends to crumple when hit.
Struve's defense both on the feet and off his back to GNP are awful. We can't even tell how good his chin is because anytime he's KOed, it's by monster shots.
gravity-now there's some serious stuff
It's usually because someone is hitting them really really hard!
Archer0545 - It's usually because someone is hitting them really really hard!That, or they are a Brit.
Zedlepln -Archer0545 - It's usually because someone is hitting them really really hard!That, or they are a Brit.
tru dat
How much does a weak neck/traps factor into it? Doesn't the neck act as somewhat of a shock absorber?
Ansari - How much does a weak neck/traps factor into it? Doesn't the neck act as somewhat of a shock absorber?
Maybe, but it has to be in conjunction with good striking skills, i.e. being situationally aware, anticipating punches, and knowing how to roll with them.
^ i think it plays a small factor but andreis got some pretty big traps on him.
Poor hand placement and overall defense increases susceptibility for haymakers others would be able to avoid.
Real fighters- because somebody hit them hard enough in the right spot.
British "fighters"- because they got scared they had to get in the cage with a man.