Do you waggle the club before you start your swing? Well, if you're not careful, you can easily be changing your grip without even knowing it, which can lead to hooks that have no clear explanation.
If you are unaware that your waggle has caused your club to move slightly in your hand, you may never be able to identify why your club is shut at impact without the help of a camera or another pair of eyes.
The next time you are on the range, try to be conscious of your pre-shot routine. Are you regripping the club too much? Are you doing anything with your hands right before you take the club back?
A little waggle is good as long as your grip pressure is firm enough to keep your club in the same position in your hands throughout the swing. You see me waggle regularly, but I'm always trying to maintain the grip pressure I've locked in before I've finished setting up to the ball.
Keep it vertical!