My two will commonly get into it with each other when a stray cat shows up outside our window and they can smell/hear/see the cat but can't do anything about it. We will chase it away but a few days later he'll come wandering back...

So perhaps there's a stray nearby causing misplaced frustration toward each other.

Or it is also true that one could be sick/in pain and loses patience with the other one more quickly now. Not necessarily that he can "smell" it but that one may be in pain and just cranky. I know with my dental problems I've been dealing with lately, when I'm having a painful day, I will snap at my boyfriend because I'm painful, and not that he did anything to deserve it. A vet check-up never hurts.

Or they are just going through a phase where they play rough and are more vocal about it. Weeks/months can go by where my two will be perfect angels with each other, then they'll have a streak of a few days where they seem to want to do nothing but fight.