One miner or a group solves a hash, creates a new block, then withholds it from the public blockchain in a practice known as selfish mining. This causes a fork, which is then mined in order to surpass the open blockchain.
The group’s blockchain may introduce its most recent block to the network if it surpasses the honest blockchain in time. The group’s fork would replace the original blockchain because the network is designed to only acknowledge the most recent block. By changing the blockchain, the miners effectively stole cryptocurrency from other users.