My boy Mansa has this weird behavior when on leash. He will see a dog coming down the sidewalk, and will either crouch down or lay all the way down until the dog gets closer. Then he bolts towards them. Usually if he makes it to the other dog without me getting him away, he will slow down, sniff, and then we are on our way.
He is NOT aggressive with other dogs off-leash, or even if he meets dogs on-leash. We go to a dog park every week where he plays with big and small dogs with no issues (running, wrestling, fetch, etc). I don't think he is trying to be aggressive when he does his crouch-down, but I am sure that it worries the other dogs as well as their people! It does kind of look like he's getting ready to attack!
Any advice for stopping this behavior?