What causes this?

[Dominantly]

So you didn't separate her from the background you in the edit you just posted?

Nope.. I opened it up in PS elements and segregated her from the background with the lasso tool, then I hit the background with Topaz denoise.. Then I saved that and opened the photo up in Lightroom 3 beta (free version), and then used the adjustment brush and quickly traced around her face with the Burn setting and filled the floor in as well.. I did this about 5 times and then went over her face with the clarity brush, and saved it.

I didn't really spend alot of time on the edging, I used a fairly large brush head to hit it quickly, so with more time you could get it blended a bit better.:thumbup: