I think You're wallet was compromised, I have read this kind of thing a year ago. where you will send ETH on a certain wallet but any amount you sent will also be sent to another address. this is another way to scam people. they put a large amount of token inside that wallet and they let you have it with its private key, the moment you sent any amount of ETH to that wallet it will automatically transfer to another account. That's how I remembered it. maybe this is the same on what is happening to you right now.

You can read more answers here from your question: https://ethereum.stackexchange.com/questions/34143/my-ether-has-been-automatically-sent-to-another-address