It is decentralized. It's also open source so anyone can alter the code and create a new fork. There have been LOTS of Bitcoin forks, it's just that most of them have barely any supporters. Forks like segwit2x has a lot of influential backers, so it' being somewhat recognized.
In the end, the users will decide which chain is the main one, not the one who made the forks.
I have seen this mentioned in a couple of other posts -- that there are actually many bitcoin forks that have been done. However, the only ones I know about are Bitcoin Cash and Bitcoin Gold. So where are these other forks? Are their coins tradeable like BTC or BCH? Is there a list of these other forks posted somewhere?