There is a known bug in FMC version 10.7. If you test the config warning on the ground by momentarily pushing forward only one thrust lever (or pushing forward only on thrust lever for whatever reason) the FMC thinks there is an engine failure as the difference in thrust lever angle is too big. It automaticly switches into one engine out mode and therefore uses the corresponding speeds which are usually the minimum speeds.
That is the reason why you get the ALL ENG prompt which after activation switches the FMC back into the all engine operating mode with normal speeds.
It is a known problem and you should have a technical information bulletin from boeing about it in your documentation.