You might have a bit too much reverse pitch for your thumb, causing you to grip the ball to avoid the feeling that you might drop the ball. Either change the pitch so you feel more comfortable holding the ball (may cause you to have some difficulty initially getting the thumb out early enough) or, better, learn to relax the thumb.
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Wildcard
"An unpredictable or unforeseeable factor"
(American Heritage Dictionary)
In reference to my scoring