In 2016, the Ethereum network experienced a fork that led to the creation of Ethereum Classic (ETC). The fork occurred after a hacker exploited a flaw in a decentralized application (dapp) called The DAO to steal $50 million worth of ether. The DAO was intended to be a decentralized funding platform for Ethereum projects, but the hack demonstrated that it was not yet ready for prime time. The stolen ether prompted a debate within the Ethereum community about how to best deal with the situation.
Some members wanted to keep the blockchain as it was and let the hack stand, while others wanted to modify the blockchain to refund the stolen ether. In the end, the community decided to fork the blockchain, with those who wanted to refund the stolen ether going on to use the new Ethereum blockchain (ETH), and those who wanted to keep the original blockchain using Ethereum Classic (ETC).
The fork occurred because the Ethereum community could not come to a consensus about how to deal with the hack. Some members wanted to keep the blockchain as it was and let the hack stand, while others wanted to modify the blockchain to refund the stolen ether.
In order not to split the community, those who wanted to refund the ether decided to fork off and create their own version of Ethereum. This new version of Ethereum is what we now know as ETH.
Why did Ethereum fork? ultimately came down to a difference in opinion about how to deal with The DAO hack. Those who wanted to keep things as they were believed that modifying the blockchain would go against the principles of immutability and decentralization that are so important to cryptocurrency.
On the other hand, those who supported refunding the stolen ether believed that it was necessary in order to maintain confidence in Ethereum and its applications. In the end, both sides got what they wanted, and we now have two versions of Ethereum: ETH and ETC.