I'm sure image comes into it too, but I think part of it is simply the desire to have your fixed blade accessible while wearing a large pack. The inverted carry may be the only way people have seen this achieved and therefore assume it is the best way to accomplish what they are trying to do.
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Victory awaits him who has everything in order — luck, people call it. Defeat is certain for him who has neglected to take the necessary precautions in time; this is called bad luck. Roald Amundsen