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.
oh I see. actually I have the same question like OP has. but I don't want to ask it here. then finally there are someone create this topic (OP Thread) now its give me light and soon to know what else information I got to learn. but I need more information about segwit, I got an eye on this topic to learn more. thanks anyway.