If your character is suffering from the Burning debuff, and you select [attack / skill / magic] to attack an enemy, the character's ATB gauge will instantly fill up to max (or fill up to a certain point based on your "Speed" value) after attacking the enemy.
This causes the character to be able to attack the enemy continuously without any consequences other than getting the burning damage which is around 70~100. The only way to stop the ATB gauge to fill up instantly is to select "block" or "run".

Ref: https://clips.twitch.tv/BombasticBlindingJuiceRiPepperonis-czVKKsU3LKG2E7Xa