Great video,
In my view this is simply the diaphragm,
You also see it in the sagittal plane,
As the ribs pass around behind the liver you often see an echogenic line protruding into the posterior edge of the liver. The diaphragm is not a smooth structure, it has to sit between the liver / lung and posterior ribs. The diaphragm, rather than being a lovely smooth echogenic line is often more irregular.
I think this is what you are seeing.
I see it often and next time I see it, I will take some axial and sagittal videos and post them here.
I am sure it will look similar to your video.
So in summary, I think it is a normal appearance.
Steve.