If you guys are familiar with Bollettieri's Sonic Serve, I think it leads to a another possible explanation.
I think the key is how far you upper body ends up in front of your hips.
Following the Sonic Serve, the hip stretch leads to the hip snap which is equated to the action of a pole vaulters pole. If your torso snaps upward from the leg drive and hip stretch, the momentum of this is going to eventually carry your torso forward and your shoulders will end up out in front of your hips. The kick back of the back leg is necessary to counteract this so that you don't fall on your face when you land.
A more simple explanation could be executing the leg drive at enough of a forward angle. If you watch a clip of Pete's serve, it almost looks like he's falling into the court before he launches. At this point, the body is pretty much in a straight line, but angled forward resulting in the upper body being further into the court than the rest of the body.
Both explanations result in the shoulders being ahead of the feet and hips which is going to require the kick back as a counter balance.