Why is my guinea pig doing this?

My two female piggies usually get along, meaning they don't scrap or anything, but every so often I'll hear a growling/rumbling sound in the cage, and it's always the older one (slightly, i think they're only a month or two apart) growling and vibrating and walking slowly towards the other, kind of waving her bum back and forth. It looks like she wants to fight and is trying to intimidate her, and it works cause Roxy always runs away and makes a whining noise. Is this just a hierarchy kinda thing? Should I be worried? Thanks in advance.