It’s the Reverse Brake Assist. It's got this cool feature that slams on the brakes if it spots something in your way - just in case you miss it! If you prefer to manage braking yourself, just disable the feature or accelerate to override it.
Reverse Braking Assist can definitely be annoying at times, but honestly, it’s come to my rescue more than once. Instead of just switching it off, I’d recommend tweaking your garage setup or surroundings a bit. Something’s probably causing it to get confused.