Based on the descriptions I've read so far, it sounds like the drones didn't give enough space around the crane boom, which it seems like they avoided. That's not to make an excuse. But it's a different defect than failing to detect the crane boom.
My theory could in theory hold if a specular highlight off the boom arm created some type of confusion I suppose, however, I posted it as just thinking aloud, I don't have much faith in my own theory at large.
(my degree is in digital imaging technology so, fun thinking problems for me :)
