Mimics copy instinct and muscle memory - it's kind of fuzzy where the line is of what, exactly, is copied, but generally things that people do automatically, like breathing and kicking when one's knee is hit with a hammer, are copied over seeing as they don't involve the brain so much. Not jumping in a pit is an instinctive survival mechanism, so mimics have it.
...well, that's what I'd say, if I were writing the canon.
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