It all depends on what you were shooting. A scene with dark and light areas without dominant midtones could show this pattern normally.
For instance, my image of a chess board on the front page would have that histogram if I pushed the white and blacks.
Post an example and we can help more.
To add to Gary's helpful comment, assume you took a properly exposed photo of a wall that is black on one half and white on the other half. The histogram of that image would be similar to what you described.
As he mentioned, provide the image and you'll get more helpful responses.