It feels much more natural to hold the camera horizontally, so it's not surprising that people forget to frame vertically. Start by framing vertical subjects vertically. Consider the option of camera orientation as another tool for filling the frame and minimizing wasted space around the subject. If something other than the main subject is the sharpest part of the composition the viewer's eye will rest in the wrong place. The lens is focused on the wrong part of the composition.
Shutter speed is too slow for hand-holding and camera shake results.
The subject moves. The operator jabs the shutter release wobbling the camera. Press the shutter release gently. One of the traps with the rule of thirds for auto-focus cameras is if the subject is not in the middle of the frame, the main subject may not be in focus. Most auto-focus cameras have a focus lock facility, which you should be confident using.