There's also a small button just above and to the right of the 4-way pad (the 1-2 button). Pressing this will lock out the 4-way pad (mode-wise) and place the camera in selective AF where you can choose the spot in the optical VF which the AF will use. That can still give you grief if the 4-way is accidentally bumped but it will only throw off the AF sample location and not affect major things as in your case. Early on, I found out about this issue when my camera suddenly began taking shots seconds after I pushed the shutter release. Only later did I find out that the camera had switched to the 12 second delay mode (good for self-portraits but not good when you're shooting action).
The same button also selects which SD card you see on the LCD screen in review mode (when you have two cards).