Turret active skill
Its because turret actives aren’t like Ranger F2 skills where you directly make the turret do the attack. Rather, it simply makes the next attack (or attacks) the turret does naturally change to the active effect shot instead of the normal one. With a time limit to use said attack before it switches back to normal, so you can’t just preload an overload attack and let the cooldown run out and get another one right away.
Since the turrets won’t normally attack if there aren’t enemies nearby, you don’t get the overload when no enemies are nearby either.
Its definitely intentional, since they’ve done things like reduce the time limit before an overload shot “goes away” (you used to be able to get Rocket Turret to use 2 knockdown shots it you used it right before it fired, same with Net Turret IIRC).
WHY it works like that? Who knows. Just one of those strange things about the game and different skills. Some work this way (turrets, elementals, certain Necromancer minions IIRC) while others will use the ability directly when you use their active skill (other minions, spirit weapons, ranger pets).
Also, bomb kit is what you want.