What people usually refer to when they say that Bitcoin is decentralized is that the system is built in a distributed way, where there are a lot of miners, nodes, and blockchain copies around the globe. What that means is that no entity can take the entire network down. There's no single point you can attack to cripple the network, so it cannot be controlled by governments or similar entities. Everyone in the network just follows the same set of rules to keep the system running.

The rise and fall is dictated by supply and demand, which are further dictated by factors such as global developments and speculation.