Sounds like the guys arrow was off, hit hard bone, or broadhead didn't open just right, some or all. With a mechanical there are just more things to check. More to go wrong if it is not checked. Most mechanicals come with a practice head that is not the actual one being used on game and is closed not allowed to open. All products have a potential for defects. Molds making products get worn over time, process, or components could change, or weight could change, one blade could be more heavy, causing incorrect flight path. Have you ever noticed some arrows just fly more true than others, some ammo patterns are different, even if exact same kind?
Would it be best to shoot the broadhead being hunted with at a target even though it is mechanical just make sure it flies true? The broadhead should be cut out of the target as to not damage it, inspected closely, and blade sharpness checked.
Even a slight weight difference of a broadhead can make an error not fly true. An elk shot over a corn pile with a pre-arranged set-up would not be the same as one walking through the woods on different trails. Lot more possibilities for human error. We should check every arrow and broadhead combination before shooting any animal. Even then some testosterone rutting bull with all that adrenaline would keep going on with a shot that would take the usual animal down.