There is always a reason for failure, but it may not be visible.
There's mechanical failure: physical damage to the casing or leads that breaks the electrical connections. Repeated stresses on the leads at the casing can break them off right there. Can also be fractures in the casing itself, which you may be unable to see without high magnification, and those fractures could propagate either into the die itself, or across the wires or bond points to leads or die, and pull them off. I don't know how likely either is, relative to other possibilities, but lead breakage is pretty danged easy to get.
There's also static electricity, which can cause immediate failure, but usually doesn't in today's electronics. But it *can* cause latent damage, which can fail later on quite suddenly, or can cause strange behavior. *Any* handling of electronic components, even with ESD precautions and equipment, can cause ESD damage, though it is much more likely when precautions aren't used. Since it's not likely people will use precautions when handling the hall connector to plug into the controller, either at factory or in shipping/handling/packing, or installation, it's easily possible wiht most of the hall connectors out there to touch pins which will transfer any high voltage static charge on the finger to whatever pins that is, potentially damaging the halls attached to them.
The same thing can happen when the halls are being installed in the first place, at the factory (or during user repairs or mods), and is probably even more likely, as neither place is likely to use any kind of ESD protection.
Then there's Murphy and the Gremlins, who like to sneak up and break stuff at random times just cuz, but I tend to think that this is caused by one of the above.
Most of the failures of parts can be traced back to how they were handled during installation (or elsewhere in the factory using them), in my experiences of failure analysis, when I worked at Honeywell a couple decades ago. Most of the rest are in how they are used in a design, where some factor was unaccounted for in the design, and they are overstressed electrically or sometimes mechanically. A very few may be actual part-manufacturing-defects.
Since most of the sensors in these motors are probably "chinese clones" of the "real" parts, I would guess but don't know that the manufacturing process of the sensors themselves might be a source of problems, but I would still bet more on the mechanical mishandling of them, or insufficient ESD precautions, even at the parts factory and distributors between them and the final usage point, than on manufacturing defects themselves.
