I suppose the question is how a slave nobody wants got to a frontier town on the ass end of nowhere, but she could have been a laborer that somebody sold to buy some extra goods to take back with them. Like a sociopathic Spice and Wolf.
Well, it was just general idea, and I didn't strictly thinking about it "in game setting", I just pictured her as older woman that didn't had luck for her previous masters (and it shows one way or another - for example she could have scars) and at this point she's just not a good deal or have some kind of bad reputation.
Saying that, there are few things that come to my mind, something as simple as - since she got older and was abused her work efficiency got lower and lower so previous owner decide to sell her. There are also was that bit (if I remember correctly) about "unruly slave" when slave refuse to obey their master, it could be used somehow in that example too.
There are other options too, previous owner got bored with her since she got older and sold her, but since she was used for "entertainment" only and is overall clumsy and not good with other jobs the new owners just return her because they don't have patience, and that's how she was passed by people and ended up in the "end of nowhere".
Overall, there is a lot of space for creativity here, I just liked the idea about mistreated milf slave in whose life we can bring some happiness (one way or another), at this point I didn't even think what kind of race she could be... maybe just human...?