The DAO hack happened:
an experiment on Ethereum called DAO (Decentralized Autonomous Organization) went horribly wrong and many investors lost a lot of Ether: estimated around 15% of ETH in circulation at the time. More specifically, it was hacked. The exploit was not on Ethereum, but rather the DAO which was built on top of Ethereum.
The ethereum developers just wanted to reverse the huge hack, it was accepted by consensus, but a small group wanted to retain it as it was before the hack, as they “believe it goes against their core values”, hence the fork (or split)