For the Area Nax, sometimes trainers will want to rebattle you.
Is it steps taken, amount of time passed or specific time of hour/ day?
>To Add: Whenever you enter a Route or Cave area that has Match Call Trainers the RNG checks to see if they want a Rematch, Gym Leaders are Different, they appear to be more random.
This is in reference to Emerald, but I'm sure the same applies to ORAS. Convert it if you don't like the answer.
Hope I helped. :)