Well, I think he has A♥:A♣: because almost all tight players will mini 5 bet with only this hand. He thaught that if he mini 5bets, maybe the other player would call some part of his range, like QQ-KK because BTN wants to let some bluffs in the villain´s range.
Also, he could push some other hands, because he thinks BB could be using his tight image to bluffing in this situation.
So thats my opinion!