Firmware Version 2.0.8 for the EOS-1D X fixes a phenomenon which occurs when the 'Shutter button half-press' menu item, configured in 'C.Fn5: Operation' > 'Custom Controls Menu', is set to metering start only. Depending on when the shutter button is half-pressed, it may initiate AutoFocus.
To download the firmware update visit the Canon website
To download the firmware update visit the Canon website