Own Account Driver:
Most likely scenario is it somehow interrupted the air supply down the red line when the susie stretched.As a safety mechanism, if for example the whole trailer became detached to stop it rolling off down a hill, if the pressure from the red line goes there’s an emergency relay valve in the trailer that sends full trailer tank pressure to the brakes as if it’s an emergency stop. Possibly the coupling on the trailer is worn, allowing air to escape when it’s pulled about or if rusty the coupling may not even have been on properly which is a good reason to make sure you can twist the collars after connecting up.
I don’t think it was an interrupted air supply as it wasnt a tight turn because if it was it would have gone in a stores yard I’d been to…
I always twist the collars round though 