The video, shared by the Twitter account @Aviationbrk, shows the pilot standing on a luggage conveyor belt with the co-pilot as he attempts to move through the plane's window.
The pilot struggles to enter the plane's window head first before the co-pilot pushes him through and guides his legs inside the cockpit.
In the video, a numerical key pad in which the pilots can enter a code to gain access to the flight deck is visible, but according to 121Pilot's resident pilot on Live and Let's Fly, the keypad wouldn't be functional if the plane was shut off.
"At least on the Airbus if the plane was fully shut down and external power turned off in the cockpit if the cockpit door was closed you might be locked out," said the pilot, who typically flies an Airbus craft rather than the Boeing 737 seen in the video. "There are some systems that can be powered on from a switch outside the cockpit but I’m not sure the door is one."
It seems that the pilots were locked out of the craft and entering through the window was likely an easier and quicker option than hooking the plane up to power to gain access through the key pad combination.