Fade turning to a pulled hook
Something about your swing definitely changes if the fade becomes a hook!
Here's another thing to consider - but not too intensely!
Do you consciously change anything? As Bob posts, trying to swing it harder than 'natural' is very likely to throw you (more?) off balance, something your sub-conscious HAS to correct, so makes auto-compensations. A different position (of arms of where you are facing) from normal at the end of the follow-through would indicate that this has happened - and what the adjustment was. A more subtle Auto-Comp (not necessarily a balance one) is done via the wrists. The brain believes (and it may be right!) that you've bot a bit far forward, so flicks the wrists a bit quicker to 'catch up' closing the face in the process.
And if you add an equipment point of view.... If you swing harder, then the face of Driver has a tendency to open further than normal - laws of physics (angular momentum etc). A shaft with a lower torque value will reduce this effect (that is what the torque value is about). If, through experience, you brain has realised that faster swings (used to) result in slices (they do in my case!) then it can also auto-correct by slowing the swing down just before impact. The shaft will twist the head back into a more neutral, or even a closed position.
Physically (for a righty).
If the swing might feel/like a heave with the shoulders and you will probably end up facing further left than normal - face simply gets closed and path stays out to in.
If the swing feels more wristy/handsy and the arms end up up further left and often lower than normal - face gets closed by hands/forearms ('correcting' too much)
If swing feels/looks 'quick but quit' then it could be either of the above or the shaft torque getting involved.
Probably worthwhile to make a conscious effort to swing 'easy', particular later in the round. Very often this produces a better strike which more than compensates for loss of head speed, so the ball goes further anyway. From an equipment point of view, a stiffer shaft is likely (not guaranteed) to be straighter as is a lower torque shaft of the same flex - though 'same flex' is pretty hard to define. If you are happy with the shaft, don't change it!
So good luck finding out what was cause - it may be that a long range session will get you to the 'last holes of the round' point and some test Drives could help you identify what the cause is. Of course, all the warm-up might just mean you'll hit them all perfectly!
Swing easy, swing better!
