Sorry if this is a obvious question, but why do you have an ATK and DEF crew? Do they both attack in a brawl or are they simply there so that you can toggle between your best ATK team and best DEF team?
The ATK crew is simply 5 pirates you choose to fight other players.
The DEF crew is simply 5 pirates you choose to defend against other players, who may attack you. NOTE: this "defending" is done automatically
The 5 pirates you choose for either can be changed manually at any time (even right before brawls too!), but the game "optimizes" your crew by default, meaning it tries to put the best pirates for either attack or defense within the cost limit.
The reason for this, is because some pirates are better at attacking, while some are simply better at defending. It lets you "pick-and-choose" within your pirate crew to maximize your chances of winning every brawl.
Hope this helped!