This dog has probably been planning an escape for months. It's been quietly sabotaging all the other cages and changed the paperwork so it didn't end up in a walled-off section with a plastic barriers. (That poor bulldog was supposed to be in the cage!)
Finally, the moment arrived, and the dog made a run for it! The cage was just a clever ruse to cover the escape. It was all going so well! But the plan didn't count on a resourceful human actually paying attention. Foiled!
Next time, though ... there's always next time.