Your getting too much axis tilt (spin), basically the hand turns too early and your thumb is exiting on top of the ball.
Axis tilt is determined by the position of the thumb during the release, some things to change your axis tilt are by how you Cock the wrist, cupping the ball (Keeping your wrist firm at least)
Study the Analysis of the modern 10-Pin Bowling Swing and Release video here:
http://www.bowlingboards.com/threads...ng-and-Release
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