One theory is that loud buzzing-like noises can drown out their ability to communicate. So some of their communications are through pheromones but they also use a buzzing dance to communicate nectar locations. So the theory is that things like lawnmowers and weed eaters make it so they can't communicate and that makes them defensive. This is usually due to the volume and had to be close to the hive. It's possible the higher pitch of the drone more closely aligns with their buzzing and doesn't need to be as loud to be disruptive.