Besides grip...a too steep downswing can cause this, along with a tilted forward body at release(which causes the steep downswing).
The pros talk about a long flat spot.
On the morichbowling.com website they talk about how to creat a flat spot in ones delivery.
ALSO...Dave Davis he of the super form and PBA hall of famer says flat releases come from having a body posture that puts a proud chest slightly behind the slide knee. Or as he also says....17 degrees off vertical.
REgards,
Luckylefty
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Open the door...see what's possible...and just walk right on through...that's how easy success feels..