fighting cats

i have two neautered tom cats about 18ths old,one has starting attacking the other for no reason,they are well fed and looked after,i dont understand why as they are brothers and got them at same time,

At their advanced age, the attacker might be suffering from the onset of dementia and consequently he is not entirely responsible for his actions.
   
He might also have some physical issue that is starting to manifest.
   
If this were my cat, we'd be making a trip to the vet just to be sure he doesn't have a health issue.
   
If he has dementia, well I guess you just have to suck it up and deal with that. Goodness knows when we all get to be your cat's age, we may all lose our marbles as well!
   
Good luck, hope this works out for you and your guys.