She's a variant from before the timelines were merged. She didn't branch from the Sacred Timeline, but was original from one of the alternate timelines that were destroyed/merged that somehow survived. She may be an alternate Loki in some way, like Odin rescued an orphan Ice Giant girl instead of boy. Odin gave her a different name. But when the Sacred Timeline was made, the Timekeepers decided on a boy orphan being rescued out of the possibilities. So now she's resentful of Loki, because he took her place in the timeline, so she wants to wreck havok to restore multiple timelines so she has a place to exist.

While they can be created via time travel, I think there's also the possibility that the timeline wants to split into multiple timelines, it's natural state. After all, the story is that there were originally many timelines, which the Timekeepers merged. So a single "sacred" timeline is unnatural, trying to impose order on a chaotic natural world. But the timeline keeps wanting to split. So when there's events/decisions where a person could go either way, both events occur, one perpetuating the sacred timeline, one creating a variant timeline. And the people most likely to create such splits are the impulsive, unpredictable types, which is why Loki is one of their most common variants.

At first, I thought Ravonna must be having some secret plans she doesn't what Mobius to know about, maybe a power grab for the TVA. Maybe she's in league with "Lady Loki," who brings her trophies on their secret meetings. But another thought: what if the TVA analysts themselves can go variant at times, and are sometimes reset? What if this isn't the first Mobius? There were past Mobiuses that did missions and brought back trophies, and they developed too much free will and were reset. It would explain why Mobious doesn't remember leaving the coffee rings on Ravonna's furniture.