"How a Split Happens

Bitcoin’s heaviest-chain rule is what makes this dangerous. If enforcing nodes and sympathetic miners begin rejecting blocks that contain non-compliant transactions, and those blocks carry valid proof-of-work under legacy rules, two competing chains emerge. Enforcing nodes build one chain. Legacy nodes follow another."

https://news.bitcoin.com/bip-110-pushes-bitcoin-toward-august-fork-deadline-with-only-5-eh-s-signaling/