I used Sparrow wallet and it physically does not allow to set that high fee of 0.7 btc. No way I could set it. Could it potentially be a hack? If so it is odd that the entire change was used as a fee. Very strange case.
It is possible, especially if you are using wind0ws OS that can easily get infected with some malware or keylogger.
There is also an option you downloaded and installed malicious Sparrow wallet or it was some type of weird bug, but I think there is also a way to manually increase transaction fee.
It does sound strange for hackers to send so much money to a miner.