Weird habit - a female mounting a female

[Hanako]

One of my adult females developed a weird fetish with another adult female's back. She constantly climbs on another female from behind. I am aware that females do it sometimes when they are pregnant, or asserting themselves (who is the boss). But this looks different. She runs to this female (always this one particular female, not the others), climbs from behind at her (standing on her back legs) and then they sort of make themselves comfortable and fall asleep like this. If I take her off and put her at the other end of the enclosure, she is running around until she finds again this female and does the same. She doesn't appear to be doing it in aggressive manner (it doesn't seem to be showing off her power) and she doesn't do any sexual movements either. She doesn't imitate sex, doesn't make noises, doesn't open her mouth and doesn't do pushes like a male would do. Does anyone knows what is this? What is the purpose of it, why a female on a female, and why it always have to be this particular female climbing on that particular female, never on anyone else?