The escape slide "girt bar" normally engages in the floor hooks when the door is closed, normally electrically and can then be selected to deploy or not when the door emergency T-handle is pulled from the inside.
However if you look closely from the start of the video you can see that the door is not fully closed (blue paint line out of alignment). This could cause the "girt bar" to partially engage and pull the slide pack from the door when opened but then allow it to fall free.