Trailer safety, double check

This week at my work we had a power company type truck towing a construction type trailer that would carry a back hoe on it. Just read, big, very heavy trailer. Well it is going down the street and the trailer unhooks itself! The trailer, jumped a curb, and then plowed into parked cars. Just the force of the trailer pushed cars out of there parking spots and into others. Thank God, no one was hurt or killed. This spot is a heavy foot traffic area, could have been real bad.
This happened because no one double checked the pintol hook to make sure it was down, locked and pinned. It looked like two of the cars will be totaled.
Slow down. Look....
One of the few, Frank

What happened to the safety chains and did the breakaway system not engage?

I have to wonder about some of those construction trailers out there that never pass through a weigh station. Many could be running around without functional brake systems and will never be checked by a DOT type.