Sampras had what Yandell called a "left ball position" on his toss, what I think you are calling behind him. A key to understanding this is first that the ball is not behind him, it is actually significantly more out front than your toss - Sampras was quite athletic with an impressive vertical jump and really moved up and forward into the serve. And second, Sampras had a lot of body rotation and a fair amount of spin even on his flat serve. That leftward position allowed him to generate a lot of topspin, but it is a very athletic position that most less explosive people have difficulty duplicating.
In your video, it looks like you're pretty much going straight up and down with your legs. That may be the camera angle, but you definitely don't have the forward spring of Sampras. So when you attempt the "left ball position" at the end of the video, you basically look like you are falling over backwards.
My advice at the moment is to give up on the radical left position and work at more leaping up and to the ball while ending up landing in a balanced position on one foot in the court. The falling over backwards isn't doing your serve any favors.