https://support.blockchain.com/hc/en-us/articles/210353663-Why-is-my-bitcoin-address-changing-
Our wallet is built on an HD (or hierarchical deterministic) framework, which has a different method for address generation and management. Each public address your wallet generates stems from your wallet's xPub (or Extended Public Key). Once your public address receives an incoming payment, a new address will automatically be generated and display when you click on receive.
If you use the same address each time you receive funds, it becomes easy for anyone to track your entire payment history. This method of address generation improves privacy by automatically presenting you with a new address when you’re expecting payment.