Lots of Rotties do this. This comes from their herding side.
Baxter does this to me often, as part of a game...he does it to our other dog too...and sometimes dogs he is playing with. He stays still...crouches down...gives the eye...and then does the rushing thing.
See if there are any herding tests coming up in your area...sounds like your dog will be good at it.
To stop her from focusing on another dog, while out for a walk...just use the "leave it" command.
Gina