This is usually caused by lens distortion. In an ideal pinhole model, straight lines stay straight, but real camera lenses use glass elements that bend light and introduce some geometric distortion.

Lens designers combine multiple elements to reduce it, but most lenses still show a little barrel distortion (lines bow outward), pincushion distortion (lines bend inward), or a mix of both—especially near the frame edges. Wide-angle lenses tend to make this more noticeable, even on ordinary consumer cameras.

So if a horizon droops near the edges or a fence looks slightly curved, the lens is the most likely cause rather than the scene itself. Higher-end lenses generally control distortion better, but no lens is perfectly free of it.