burly, I want to answer your question, but I also want to avoid saying something that applies to a chunk but not all golfers, because these types of backswing length issues are varied and apply differently to different golfers.
This is a relatively good answer for a lot of golfers:
Originally Posted by shortstop20
If the arm swing keeps going after the shoulder turn has finished, the arms can get stuck behind the body during the downswing. This guy is guilty of that. I'm working on lessening my right arm bend to keep the arm swing shorter.
Basically, if your torso turn stops but your arms continue, several things can happen. Your right elbow can over-fold. It can move too far behind you. It can lag behind your torso unwinding on the downswing, or kick out the elbow flex prematurely. It can cause the pressure points to sense the sweetspot in the wrong place at the wrong time and steepen or shallow more to get off-plane. Some people will re-grip the club at the top when they "over-swing."
So while I'd like to answer your question with details and specifics, it's difficult because "overswinging" is so vague. Phil and Bubba don't over-swing because they're able to get their arms down fast enough (and their torsos turn quite a bit, too, so their true amount of "overswinging" is pretty minimal).