The joke usually goes "When is a door not a door? When it's ajar!" This panda has a new way of looking at it: a door is not a door when it's a swing.
This cute little cub digs its claws into a gap in the door from its enclosure and just hangs out there, like a fluffy little doorstop, until a human opens the door and then "Whee!" The panda gets a brief moment of swinging across the step before getting slightly smooshed against the wall. (Maybe this panda is actually half-cat and likes being in narrow spaces?)
It's also possible that this is a crafty panda that's attempting to open the door — you can see it tugging on the door after one of the keepers walks out. If you want to hide your diabolic plan, cuteness is a smart cover!