Every now and then, Hunter Mahan, whom I've worked with for a few years, won't complete his backswing. The result is a downswing that's rushed and out of sequence. Some of it is rooted in his posture, but other times he doesn't pivot enough as he goes back. Many amateurs suffer from the same issue.
To make a good backswing, you might have been told to make a full shoulder turn. But it's possible to turn your shoulders without much rotation from the rest of your torso. This isn't the best way to think about how to swing the club back. Instead, allow your arms and shoulders to start the takeaway, but then let your pelvis rotate away from the target to complete the backswing.
The golf swing is a circle, and your upper body has to rotate away from the target around a single axis. To make this happen, focus on turning your pelvis, not your shoulders, and you'll sequence your swing better.