Auto-contribute is enabled by default when you opt-in to ads in brave://rewards. You have to manually disable auto-contribute. Also, once disabled you have to exit the browser and restart for the change to take affect.
Brave has a project in the Planning stage to notify users before any auto-contribute is sent. They will be able to confirm or deny.
https://github.com/orgs/brave/projects/28#card-59081771
I don’t know why they just don’t make the default off instead of on! There was a Feature Request a couple of years ago to do just that and it didn’t get one vote! lol