There’s no Diablo 3 hacking involved, nothing as sinister as that, but some extra weight on your mouse button while you go about your normal day could have you levelled to Paragon 100 in only four days.
Using the PvP map and a Witch Doctor, Zombie Dogs can be spawned then killed to gain XP, and assuming you have the right gear, the dogs can be pumped out at a phenominal rate.
As of writing this bug has not been fixed but hopefully Blizzard will hop right on this. Why would anyone do it? No idea really, but gamers are usually motivated by the fact that it can be done.
It’s quite mental as this video illustrates. Now please sort it Blizzard.