Sudden onset of excessive grooming/fur pulling a cage mate

[Siphonophorae]

I've had my two female chinchillas, Effie and Elenie for over two years, and had no issues out of the ordinary.

Yesterday I heard one of them grumbling a little more than normal, and making a 'grunt squeak' noise a few times. I went to investigate, only to find Elenie grooming Effie. I'm used to them doing this and often they get a little carried away and occasionally keck at one another, but this time I noticed a little tumbleweed-like fur clump in the cage.

They seemed to leave each other alone after that, so I assumed it was a one off, but Elenie has been at it all day.
The confusing element is that aside from the occasional grunt, Effie doesn't seem bothered at all, she isn't fleeing, screaming, barking or trembling.

There isn't an alarming amount of fur that I've removed from the cage, but three loose balls of fur that have clearly been removed from the roots as opposed to being chewed off.
A few times I saw Elenie in a 'mount' position, with her paws on Effie, but most of the time they're sitting side by side snoozing together, and Elenie will start enthusiastically grooming behind Effie's ear, her side or her back.

They're both out with me now, exploring, gnawing, eating normally, drinking normally, so I assumed it was perhaps boredom. Except I just saw Elenie doing it again whilst they were both napping.
Has Elenie developed some form of neurotic bad habit, like a relaxation/self-soothing behavior that's become a little obsessive?
Her whole manner isn't that of frenzied aggression, more focused enthusiasm.

Also I noticed Effie reciprocating a few times, and grooming Elenie back, which suggests that Effie clearly doesn't feel threatened or under attack, I just wonder why Elenie feels the need to get so over zealous with it all of a sudden.

Of course now my mind is racing with horror stories, and images of waking up to find an injured chinchilla or fighting or other such nightmares. Sincerely hope this isn't serious.

 

[Cuddlebug]

I see one of two situations here. One, they, for what ever reason, need to reestablish who is the dominant chin or two, one is actually a male and you are seeing mating behavior which can get aggressive if the female does not want to mate. In the first case, keep an eye on them. If they it continues, you may need to separate them to prevent them from hurting or killing each other. For the second case, unless you intend to breed them, you will need to separate the, Males and females can fight as bad or even worse than same gender pairs. Regardless, you should check their genders or take pictures and we can check for you. Just because they have never had babies so far, doesn.t mean that they can't. Also, this may be just the first time you have seen them mate. I am not trying to tell you that you don't know what you are doing. I am just throwing out the possibilities for you and anyone else in the same situation.

 

[Siphonophorae]

Thank you so much for your reply. They are definitely both girls so I think your first suggestion might be whats going on. They seem to have calmed down a bit, still over grooming but not quite as constantly. I bought a much larger cage recently and I'm stuffing it full with things. As soon as they get more confident exploring it when they're out of their cage, I'll move them in. Hopefully they'll be a bit calmer when they have their own space and are not constantly under each other's feet.