Please be reminded that the Arena battles or matches are AI controlled. The game or system decides each unit's action based on a set of parameters during the match. It is more easily explained that Brave Burst execution is random or AI based and the developer nor the end-user has control over it while it is happening.
This is the current system for the Arena feature of the game and is constantly improving or evolving as time goes by, in this regards some future changes can be expected if the developer decides to improve the mechanics along the way. So far this is what works for the game and the user.