On a general level, the lighting is diffuse and the focus loose. The pose is one that implies a shared moment. This gives an implied intimacy, even were it just a random pose.

However, it's also showing a shirt pulled aside, which heightens the sense of intimacy.

Overall, while it's definitely not a sexual image, we're partially conditioned to connect intimacy with sexuality. This makes it hot rather than just another picture of the same person. She's a gorgeous woman, but if she was just standing somewhere, the only "hotness" would be in the mind of the viewer since such a pose has no triggers to pull.

But this is a very sensual image. I just wonder if the person that snapped it knew what they were doing, or just got lucky.