Normally this is from a jumper being placed on the power-on jumpers or the power switch has being shorted. The ATX spec of system power-on uses a monetary short to ground (logic 0) to both turn on the system and turn off the system. If this is always at a logic level 0 the system will turn on and after the BIOS gains control of the system the logic 0 of this input will turn the system off. Please remove the jumper or switch connection from the PW_ON pins and monetary short these two pins, if the system runs correctly check the switch for shorts.