Why is this NPC following me?

Solution 1:

So after doing some research generally it's either because of or related to mod issues or simply a vanilla bug.

It's hard to give you an specific answer for your case because no idea which one is your case. That being said, most vanilla cases I found were related to having bounties and happened with random NPCs (which could be fixed by simply clearing all bounties) or were related to specific NPCs (fixed differently for each case). And in the case of mods it was always an specific mod issue (also with a particular fix).

PS: I know you didn't ask for a fix, but assuming you do want to fix it you can always try to just wait, sleep, reload a previous save or well, kill her lol. Or you can always try console commands. Anyway here are some sources: 1, 2, 3, 4.