Why is the Bitcoin Gold Hardfork happening?
It's just a group of people trying create more money and gain control of their own blockchain using a hard fork.  

My guess on why they aren't starting their own blockchain and just forking Bitcoin's is that Bitcoin already has huge name recognition and they probably already own a bunch of Bitcoins to begin with.  If they fork the Bitcoin chain with the name "Bitcoin Gold" they can basically ride the coattails of Bitcoin and possibly create an altcoin with significant value (making money out of nothing).  This is nothing short of fraud, and anybody who ends up buying "Bitcoin Gold" coins is essentially just giving them free money.