Firstly, the Bitcoin is not actually sent back to your wallet. Your wallet just "forgets" the transaction and reverts to a state where the transaction was never sent. However, that does not mean that everyone else on the network has forgotten your transaction as well and someone could be rebroadcasting continuously.
However, there is a bigger issue here, the addresses in the transaction changing. This should not happen unless you are sending the Bitcoin again. If this is happening, that means that someone has gotten your private keys and compromised your wallet.
No, I mean that the address I was sending to changes every few days or so, not that my address changes.
Yeah it wouldn't have gone out again on its own unless you ( or someone else ) sent a new transaction. Check your history and see if there was any recent transactions.