Background: I have two guinea pigs, Penny and Rosie (both female, both around 3 years old, neither spayed). I got Penny in May of 2017, and she was a single until January 2018 when I got Rosie. (My vet told me to give Penny a few months to get used to me before adopting a second pig, so I waited until the semester break.) They've never been super close friends (they seem to have different ideas of what counts as enough personal space), but they're civil, and I know how important it is for guinea pigs to have cage mates.
Overall, their squabbles have been very minor: Rosie gets in Penny's space, Penny snaps at Rosie, and Rosie backs off for a while. But for the past few days I've seen a lot of squabbling that looks like an introduction all over again. They chase each other in tight circles; Penny snaps at Rosie repeatedly, and Rosie refuses to budge. There's a lot of teeth-chattering from Penny and angry rumbling from Rosie. All this happens two or three times a day and lasts several minutes each time. So far there hasn't been any real fighting.
My questions are: Is there a risk that Penny and Rosie will need to be separated? How closely do I need to watch them? And why is this happening now, when they've lived together for almost two years without any problems?