Wow.
Likely a frozen bearing that finally got so hot it snapped the axle, judging by the smoke.
Could easily have killed someone! When I lived in CA circa early 90's in Interstate I-280 a rear wheel from a sand truck's trailer came off and struck a Caltrans worker, killing him from the massive trauma. A few years earlier a similar incident happened when a big truck wheel, also from that same I-280, traveled down an off ramp and hit one of our traffic signal cabinets...LOTS of damage.
Just in the last 3 or 4 years I have witnessed several tandem wheel cargo trailers going along, pulled by a pickup truck, with a shredded tire or a squealing bearing, the driver not checking rear view mirrors periodically, something that one has to do when pulling any kind of a trailer.
Thanks for the share!