Vertical lines are going to tilt inward ("keystoning") any time the camera isn't level with the ground. The effect is easier to get with wide lenses.
It's pretty easy to "fix" in Photoshop. I put that in quotes because I don't think it's always desirable to correct the perspective. When the keystoning is severe, it looks really unnatural to correct the perspective (and it also requires heavy cropping of the image). A good rule of thumb is to only correc the pictures you take with the specific intention of correcting, or ones where you try to keep the camera level but end up tilting it a little and producing some keystoning.
Your photo is a good candidate for perspective correction. I took the liberty of doing it:
(I only uploaded your photo to my server for this thread, but please let me know if that makes you uncomfortable, and I'll delete it)