Tallullu is right on about what the behavior means. As long as BooBoo is allowing the mounting then that is good. It is helping them establish who is going to be the "queen". Doesn’t mean that they won’t equally care and groom each other later if they become extremely bonded, but it is just how they establish the rules. (Most of the "rules" are no longer needed in domestic rabbits, like the top bunny gets the best nesting areas, etc) but they still have the instincts to establish rules.
What you should do is allow it to happen, but watch carefully. If Mable insists on continually mounting BooBoo for long periods at a time, then BooBoo could get tired of it and fight. SO here are a couple of things you can do to make it easier for BooBoo.
1. Pet Boo while Mable mounts her.
2. IF Mable is a continual mounter, meaning, she will mount longer than 30 seconds in one mounting, then gently push her off after 20 seconds, and make her take a break for a minute, before allowing her back on again.
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