I'm no expert but one of my girls does the lunging and gets quite aggressive, I think it's because she's territorial since she's a mother(not recently but the youngest is still with her) I've just left her in a separate cage with her youngest baby. My other 4 gliders are all together but I'm not going to introduce the mother because I know that she will ball up with the newest glider I bought, the rest are her daughters but she won't recognize their smell so she'll probably hurt them.
is the pair two females or a male and a female?
because I think the girl in the pair could be 'guarding' her male, if not, she's probably just guarding her territory.
I've heard my gliders doing the squeaking and then hissing before, it happened when I separated the two males(dad and son) from the girls because I was going to sell them, this was mainly on the dad's part because he lost all his girls and was 'crying' but the mother also did a similar thing when the new gliders were put into a cage next to her.
so your girl could either be grieving or being territorial.
I'm no expert and please don't take my word for it, check with someone who knows more, but that's what i think's happening.