Does it happen with any other times, like when sitting on a tripod or in landscape orientation? If I were a betting man, I would put my money on Brian's suggestion of inadvertant pressing the multi-selector. Are you using a grip or the right-arm-overhead position? Try holding the camera in that overhead position and rest your thumb on the upper back, around the AE_L/AF-L button and look where the lower part of your thumb rests....right over the top of the right direction multi-selector. Right button is UP in portrait orientation. That is one of many reasons that portrait shooters get vertical grips and never take them off.