You're getting too much lateral movement in the backswing making the weight move to the outside of the right foot. Your right knee is reacting to this weight shift.

I think the real issue, though, is the shoulder turn. In my view, it's better to think of turning the upper chest in the backswing rather than the shoulders. It's actually possible to move the left shoulder quite a way without moving the chest by actually twisting it in front of the body. This is what you are doing to some extent and it breaks the required connection between the shoulder and the chest muscles. It also tends to encourage a sideways sway (and actually narrows the swing arc so, not a good thing).

The other clue to this is that you move the club head away from its natural arc pretty quickly on the backswing and it starts to lag the hands. This also tends to happen if you are pulling the left shoulder across your chest rather than rotating the upper body. (It's one of those things that may feel powerful but actually leads to a loss of power.)

So, the solution is to make sure you fully turn the upper body. As a check for all of this; take your current backswing and hold it at the top. Now, if you place the club across your upper chest (not shoulders), my bet is that the club line will be pointing well ahead of the ball. You may think your shoulders have turned more (and the shoulder line will be different), but, as I have said, that's becasue you've twisted the upper left shoulder around in front of the chest.

Instead of doing this, keep the connection between your outer left shoulder and the chest muscles (i.e. don't move it, twist it etc). Now, rotate the chest fully on the backswing. At the top of your backswing, your chest should be fully turned away from the target and your back will be facing the target. (Of course, your shoulders have fully turned at the same time but are still basically in the same relationship to the chest as they were at the start. Note that your right shoulder will have moved / rotated more than is the case with your current swing.)

Now, if you place the club across your chest, you'll see the line is either pointing at the ball or, more likely, behind it. This is a good thing. Also, by rotating this way, note the arc of the club head. Now that you're not twisting the upper left shoulder round, the club head stays 'outside the hands' - i.e., it won't start to lag. This is what you want to create a wide arc on the backswing.

Back on the original point; because you're now rotating the upper body rather that twisting the left side around, there is less lateral movement. You should instead feel the weight transferring to the inside of the right foot.

Try hitting a few balls with this upper chest rotation and just let the arms swing naturally. It will feel different at first because you're probably not used to rotating in this way and your swing path will have changed slightly (for the better), so get used to it with a few hits. It actually doesn't take that long to get the feel for this. With a little practice, you'll hit the ball more consistently and, I'm betting, quite a bit further.