Most truly decentralized blockchain projects such as Bitcoin rely on the consensus of their communities in order to execute changes or upgrade. Sometimes the communities cannot see fit in the changes. The minority who does not want to comply with the direction that the majority chose can split off and create a separate chain. This kind of event is called “Hard Fork.”
Bitcoin Cash (BCH) forked off Bitcoin (BTC) as the majority of the original Bitcoin community did not agree that they should increase the block size in order to ramp up transaction speed to solve the scalability problem. The people who supported the block’s size increase idea then started mining their own chain and subsequently split off from the original Bitcoin’s blockchain entirely.
Another popular hard fork is the Ethereum’s hard fork where the community splitted the chain and forked off to Ethereum (ETH) leaving the original Ethereum’s chain as Ethereum Classic (ETC.)