It's usually just to give them extra time to execute their next shot. It also depends on the situation that is up hand, such as getting the shots they need to keep themselves in the game, making their opponents perform in the 10th frame just to give themselves better chances to win, or just to shut out their opponents with reasonable count. Or even if they were on their way to a perfect game, like Mika Koivuniemi did in the Tournament of Champions a few years ago after he got the 11th strike. He took a re-rack before delivering his 12th shot, just to take his time without rushing. Mika's one of those pros that re-rack very frequently ;p Also, just like you see some of the bowlers balk, it can be from any kind of distraction such as a little bit of noise coming from the audience, or even bugs flying around all over the place. lol hope this helps a little bit (:
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