It's all release related.  

High trackers will sometimes clip the thumb hole, but it usually happens for only a rotation or two before the track migrates away from the thumb.  It's usually more pronounced on low flare balls, and spare balls.  It's usually not a big deal.  If the ball is thumping at the backend of the lane then it is most likely flaring into the fingers.  In that case you need to take a look at pin placement, and probably raise the pin above the fingers.  If you are thumping down the entire lane then you should probably see a coach or have your driller watch you to see what you are doing wrong.

This can also happen to full rollers who have their equipment drilled wrong.  This is a big deal and you should get a full roller safe drill on your equipment.

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-Clint