Fortis Holding on Monday shocked investors with news that its chief executive would leave and that the head of its biggest division had resigned. The move means the fifth chief executive in under a year for the once-diversified Belgian bancassurance group that is trying to rebuild itself as an insurer. Karel De Boeck is being replaced officially because his experience is in banking, an activity that Fortis is no longer involved in after many of its assets were nationalised by the Belgian and Dutch governments in October.