Here take a look at these articles by Ron Clifton about pulling the ball.
Pulling the Ball Part 1
Pulling the Ball Part 2
He puts it down to Early Timing.
Early timing happens when the ball reaches the release point slightly sooner than normal.
He lists several possible causes:
Shortened Swing Cycle
Holding The Ball Back
Long first step
Lazy Push Away
Cutting off the backswing
Bending the Elbow
Forcing The Ball
also
The Grip
Swing Angle
Aiming at your target
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