If none of the others are coughing or have any sign of respiratory illness (sneezing, mucus from the nose, bubbly eyes, etc,) then I would guess something like heart or a tumor, something internally wrong. Remember, chickens are expert at hiding that they don't feel well. I'm sorry.
Happiness is when you can look at your chickens and smile....
Worry is interest paid on trouble before it comes due.
Flockwatching my little bunch of mutts, a favorite pastime.
Exact same thing happened to my bantam cochin roo a few months back. Just dropped dead.
I noticed gasping beforehand, like he inhaled a crumble. I checked him and found lice and mites - check yours and see if he has any and check your girls! I don't know if that can kill them but it's not good.