My 8-month-old aussie lunges and barks at every dog we see on walks. Off-leash at the park she's totally fine with other dogs. What's going on?

Leash reactivity. The leash removes her ability to choose flight or approach, so she defaults to big threat displays to make the other dog go away. It works — the other dog usually does leave — so the behavior gets reinforced every walk. Off-leash she has options, so she doesn't need the display. You're creating the problem by walking too close to other dogs. When you see a dog, increase distance immediately — cross the street, turn around, duck behind a car. Get far enough away that she notices the dog but doesn't react (usually 30-50 feet for an aussie). At that distance, feed her high-value treats continuously while the other dog is visible. Stop when the dog is gone. Repeat 100 times over two weeks. You're reconditioning the trigger: other dog appears → treats appear. Gradually decrease distance over weeks as she stays calm. If she lunges, you're too close. Back up. Most aussies need 6-8 weeks of this before they can pass another dog calmly at 10 feet. Don't let her greet on-leash during this period — that muddies the training.