Focus problem

[brmill26]

This afternoon / evening was my first attempt at taking true landscapes with my XTi and 50mm f1.8 II lens. I took about 160 shots. I discovered that nearly all of my shots with a wide aperture (roughly, less than 3.5) were anywhere from noticeably off focus to completely wrong focus.

Camera shake was not the issue, as every shot in question had a shutter speed of at least 1/100, and most 1/400 or higher. I was shooting in Aperture priority mode, standard picture style, center focus point selected for the AF.

Is this a technical problem or a photographer problem? Thanks for your help.

This shot was one of the worst - taken at f1.8, 1/200, partial metering, ISO 100. The trees weren't the problem either - there were shots taken from clear spots with the same issue.