Likely no medical reason. I mean stuttering has a genetic component and there is a difference in the brain when it comes to motorneutron coordination, but that's just regular developmental stuttering. It's good you aren't rushing her or giving her feedback that stuttering is wrong or bad. That's the most damaging part of stuttering, when kids learn it's bad and they should try to not do it. But if she's exhibiting struggle in the middle of words, a good SLP who gets stuttering can help (hard to find someone who gets it). But if she doesn't appear to be struggling and that is the form her disfluencies take, I wouldn't overworry. You want her to be comfortable stuttering however she naturally stutters and not try to modify it because that's where the problems come in.