The root cause I was given was the incorrect dust cap had been fitted to the socket on the car when the bar was installed. This meant at some point the cap had detached and the socket had gradually filled with road dirt and corroded. So even though the ball head locked into place and showed green indicators , it failed part way into the journey.
I’ve always been a bit leery of detachable bars and so always took a lot of care hitching up. On the day in question the ball head was stiff to engage, but since everything was green (and after I jumped up and down on it!) I assumed all was well.
Luckily no one was injured as a result of the failure.