I think this was probably some April Fools joke played by the #ubers gang. They went on the ladder en masse with Magikarp leads and quit, and in a day managed to rig the stats.Sounds like there's some sort of Uber cult which uses lead Magikarp as part of their rituals. Maybe there's more here than meets the eye with this little fish?
Or there was a glitch/hack. Either way, this is wayyy too much work for a sane person to do in a legit manner.