It happens a lot with dogs at the dog park here. Even my Chloe will act like that sometimes. Basically, it's setting boundries. Chloe will let a strange dog come up and sniff... once. If the dog gets too pushy and wants to keep sniffing, she'll snap at him, and tell him to get the hell away. I think she's saying, "I said you could sniff me... once! You know what I smell like, now back off!" If the dog backs off, she's fine and will keep playing.