WHAT: In 2013 Cornell researchers Emin Gün Sirer and Ittay Eyal theorized that miners could hijack legitimate blocks by hiding their own newly-generated private blocks from the main blockchain creating a separate fork.
WHY: Once the selfish miner produces a longer chain, honest miners would be tricked to abandon theirs and prefer the private/illegitimate chain which enlarges the attackers overall share of the mining revenue.
REFERENCES:
https://www.investopedia.com/terms/s/selfish-mining.asp
https://www.profwoodward.org/2016/05/preventing-selfish-mining-in-blockchain.html