Think of it as a Democracy.
Cryptocurrencies are not faultless and like everything else, always have room for improvement. The progression of decentralized currencies, like Bitcoin, are debated and decided by various entities and parties. This often leads to disagreements and hence, a separation during times of upgrades.
Such separations result in the emergence of 2 or more coins that operate on varying protocols (ie. Bitcoin Cash on 1st August 2017)
If you wish to have a better understanding of what forks are kindly refer here.