She's trying to show her position as alpha dog.
You need to train her to understand that you are alpha dog. She needs to walk beside you or behind you, never in front of you-this gives her the idea that she's in charge and she will constantly challenge any other dog she comes in contact with. If she is following her "pack leader (you)" and you don't challenge the other dogs, she won't either.
Dog Breed Info Center®, DBI
Lots of information regarding how to obtain alpha status. Some I agree with wholeheartedly, some seems a little over the top, but most makes perfect sense.