Maybe rough roads ,  try to avoid pot holes but not always possible. Must of been some warning signs that driver ignored. Vibration or some weird noises before it separated?  Everytime I stop I do a  quick circle check and look under trailer too. Fastens I always check them too. Thankfully that has never happened to me. 

You are in_town. There are people there. View?   .

Did not look old, trailer was in good shape and clean, looked cared for passing by, did not look to be derelict, just looked like some bad luck maybe.    

Shocking how old was trailer you think? I check my trailer fasteners before any big trips. Mines 12 years old now. I tow about 3 to 4K kms a year. Usually goes to trailer shop every 3 or so years. After winter when I don't need it I take it back in. Never had a bearing or axle trouble thankfully. 

Saw a guy lost his entire second axle, both rims and tires, off his Triton enclosed on the skyway bridge this am.   Entire assembly was against the retaining wall beside the left lane, truck and trailer were getting OPP escort off the bridge.   bummer start to wherever they were going.