Most likely it was the oil. Could you see the rotation on the ball as it went down the lane? Did it look like it usually does? When the ball came back, was the oil track where it normally is relative to the finger and thumb holes? If anything looked different, then you might have been doing something odd with your release. One other thought, were you throwing faster than normal? I know that for me 1 mph faster means just barely touching the head pin instead of being dead flush.
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