Often the reason for the order cancellation is because the customer as well as contacting BT and arranging service with them also contacts the provider they are leaving , to say they are leaving , this is the wrong thing to do , by serving notice themselves to the old provider, rather that just contacting BT and leave everything to BT , the old provider puts a cease on the line ( because you called them and told them you were leaving ) and this cancels BT migration order , because BT cannot migrate a line that has a cease.
The ‘switching’ process is gaining provider led , anyone switching providers on Openreach network should simply contact the company they want to join and leave everything else to them , including serving notice to the losing company, I suspect this was the case with the original poster hence why the BT order was cancelled
