Hard forks can occur in any cryptocurrency network because public blockchains, regardless of how they are constructed, operate in the same way - transactions are bundled into blocks, which are validated only when all validators in a network reach consensus on it.
Validators can propose changes to specific areas of the protocol because they understand the rules of cryptocurrency networks. However, before any change is implemented, all validators must agree on the update. If everyone agrees, developers update the network protocol to reflect the change.
The fork creates a spit in the blockchain, requiring all validators to upgrade their software in order to run the updated protocol. Nodes running the old chain will eventually realise it is obsolete and will migrate to the new chain.