I agree with the comment about the backlighting and shadow by Frank. I think those are the main factors here. Also agree that it's a nice shot. 🙂 The lower half of the statue doesn't "pop out" of the background quite so much though.
Clearly you have to have a wide aperture. Another factor may be the distance between the figure (main subject) and background. In this photo you have a differential there, with the background to the left of the statue much further away than the background on the right.
Figure/background tonality difference may contribute also. A dark figure on a light background, or light on dark would aid differentiation between the two.
Also, I think that the background needs to be in the same plane if possible, especially if the figure is a person and the background is composed of people too.
I bet there are lots of other factors.