The Casa App uses an HD (hierarchical deterministic) Bitcoin key, which generates a new address with each transaction that's made.
When sending bitcoin, leftover amounts in addresses get returned to the sending wallet in a new address. This is known as a "change" address.
Some hardware wallet devices, like a Trezor, may ask you to confirm an additional "change" address when sending funds from the Casa App.
For more information on how change addresses work, and why it's built into the Bitcoin protocol, see https://en.bitcoin.it/wiki/Change