is not uncommon in many pro golfer's swings. Here are four examples including Hogan, Garcia, Scott Hazeldine, and Tiger:
I do know that many of the announcers seem to get uptight about this head drop. My guess is because of the extra gyrations necessary to accomplish this in a CG swing. However, you'll notice in the photo, Scott (in the yellow shirt), the only SA golfer of the group, has the largest head drop. When using a SA swing, one does not have to change the spine angle significantly to hit the ball while facing it (read that as one does not have to "stand-up" to hit the ball with good contact.)
IMHO, this means that SA golfers can employ this head drop with more success and consistency than can a CG golfer. I would even go far as to say that since the leg drive causes the entire torso to drop, and since an SA golfer does not have to "stand-up" to keep from hitting it fat, that the head MUST drop for a proper SA swing, and that this is simpler than the stand-up move for CG (the stand-up that creates the illusion of the head remaining fixed.)
As for the vertical drop of the arms, I believe that this explanation is too simple, and I've already documented my own troubles with trying to accomplish this (dropping the arms straight down into the trail hip causing a loss of angles.) In reality, the arms come down AND forward at the same time. I then began an effort to move my arms properly in three dimensional space ... a somewhat daunting task as it seemed to change with each swing. It was at this point that Mac and fdb stepped in and said there is a simpler way for us amateurs via locking the arms, and dropping the trail shoulder.
Anyway, my point is that this deviation from PB is probably acceptable when using an SA grip. At any rate, as Mac has gone on to comment to me, and as others have said here before, it is probably more important in the DS to return the lead shoulder to its setup position. This is what gives you consistency of ball contact. Don't worry so much about the head ... it's not attached directly to the club.