What can cause a wheel to fall off/hub to break?

A wheel bearing failure can, indeed, cause the wheel to fall off. Mine did last week. New tires were put on last fall, and all lug nuts were torqued accordingly. This was with drum brakes. Wheel bearing failure in your case would have taken out the rotor and caliper as well. I would agree with others that either wheel stud failure or improper torque procedures resulted in your wheel going further than your vehicle.

It was untightened lug nuts, period .Unfortunately , I have done that. Miraculously , I can tell about it. Just as your photo ; all my nuts were missing, 2 studs had broken by fatigue. I had a Nissan Titan , 6 lug aluminium wheel. I rotated tires for a trip, forgot to torque one front wheel. Drove about 1100 miles on interstate, 60 to 75 mph. I felt a vibration toward the end of the trip; So when I got on my sons' driveway , I got out to remove the problem wheel. I was very shocked to find NO nuts. We pulled the wheel off and found 2 studs had fatigued off. A couple other studs has minor thread damage.We pressed out all the studs and pushed in new ones. Because my wheel did not fall off the only parts I needed were studs and nuts. My son has now driven the truck about another 100,000 miles with no related problems. So, other than repairing your damage , I doubt that anything else needs work.

First of all: I am just an amateur that makes a diagnosis based on a single picture..

Diagnosis:

My understand is that a wheel has just some limited excuses for falling of:

1. Someone didn't torqued the lug nuts correctly.
2. Someone replaced the original material (rim/nuts/bolts/studs) with something of inadequate quality.

Bottom line: Bolts/Nuts do not wear by normal use. Textbook root cause are under-torqued lug nuts. The bearing of a hub wears, but cannot provoke the separation of the wheel from the hub.

I personally would sue the last one that touched this (except it was me). And I would immediately get another mechanic, just because he mentioned that something like this could just happen with age.

Remedy

First of all: Depending on your speed at the time of the incident the damaged hub is not your biggest thing to worry about. (and yes, in most cases there ought to be replacement studs for the hub)

1. Call a tow truck and let him bring the vehicle to the mechanic of your least mistrust.
2. Think about calling a lawyer, because that did not happen by higher force.
3. Let the mechanic check the status of the remaining lug nuts.
4. Let him also check the damages your car suffered. Assuming that you where not stationary at the time of the incident I suspect some kind of under-body damage, perhaps also damage on suspension parts, engine (oil sump) or frame.

Pretty much the only reason the wheel would fall off is because the wheel nuts weren't torqued. I've never seen one fall off because of a worn wheel bearing. The stud and nut threads can stretch from over tightening.

I'd remount the wheel first and check for play in the bearing and any roughness when turning the wheel. Check the stud holes on the wheel for wear.

You can install new studs IIRC, you don't have to replace the hub. There are some cars where there isn't enough space between the hub and knuckle, so you'd have to pull the hub and replace the studs and bearing.

If your concerned about the other wheels, pull the nuts off, make sure that the wheel sits flush, and doesn't bind due to oxidation when reinstalling the wheel.