As if often the case with cryptocurrencies, the hard fork of Ethereum followed a split in the community. There was an incident in July 2016 which required the attention of the founders and followers of Ethereum, and responses to that were mixed.
At that time, attackers exploited flaws in the smart contract code of a prominent application running on Ethereum, which was known as The DAO. As a response to this, the overarching Ethereum Foundation implemented a hard fork to roll back all DAO-related transactions.
Many supported this action, but many also chose to continue running the original Ethereum blockchain. This then became Ethereum Classic.