from what i've seen in game, it's just pure luck. i haven't seen anything that says "increases chance to expose." there are abilites that have a chance to expose, so using them along with an expose attack may just seem like it's increasing the chances?
i actually did that once in a ground pvp match though. i was in a fight against two klingons, and i used my AoE exploit attack, and it exposed them both. then i split beam vaporized them. it was rather epic. but it will probably only ever happen once.