Am I being hacked, can I prevent it?
You are not being hacked.  These are not two separate transactions; it is one transaction with two outputs.  The second output was automatically created by your wallet client and sends back to a new address which also belongs to you.  This is a normal and necessary feature of how Bitcoin works, called "change". See How does change work in a bitcoin transaction?

If this transaction were malicious, it would mean that someone had access to your private key; if so, they would also have stolen your earlier transaction.  Moreover, they would  have stolen the coins as soon as they obtained the key; they wouldn't have waited for you to make your own transaction.