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Possibly because you panic. Your body doesn't know how long or hard the effort is going to be, because large predatory felines are unpredictable. You can take voluntary control of your breathing, but at TT levels of effort that's still going to be breathing pretty hard and using the mouth - you should be able to shout your number to the timekeeper, but not say "I say, my good fellow, I have the number one hundred and nineteen pinned to my shorts"
K. So why do I get all mouth-breathy and panty when I try hard?