The barbarian function of the ancestor guardian ancestral protectors (XGtE, p. 10) reads as follows:
If you choose this path in the 3rd level, spectral warriors will appear when you enter your anger. As you roar, the first creature you attack in your turn becomes the target of the Warrior, hindering their attacks. Until the start of your next turn, this target has a disadvantage on an attack roll that is not aimed at you, and if the target hits a creature other than you with an attack, that creature will have resistance to the damage done by the attack. The effect on the target ends early when your anger ends.
Suppose I have a fighter dip. What would happen if I hit a creature that rages with BA? Would the ancestors choose the creature I hit at the beginning of my turn even though anger was triggered after the hit?
If I acted quickly and made something else, would they target that?
I'm specifically wondering if you can use booming blade to trigger the ability RAW.