Leash aggression can attack even the friendliest dog. Pups can sense your tension on the leash (if he's done it before), and may act out. Furthermore, some consider other dogs a "threat" needed to be dealt with.
Tip: Work on your dog's socialization, not just off-leash but on-leash walks with another furry friend and another human leader. This can identify if he's just "protecting you" or if he simply isn't sure what to do yet with other leashed pups.