There will be employees who do not require a Manager to be set, and they will then also appear as head of the Org Structure. 

When going to change this, the option may appear greyed out -

If you are unable to leave select the option, navigate to - Administration>Company>Settings>Company settings have Employee - Manager set as Mandatory. If this is ticked to be a Mandatory field (which is recommended) untick the option and select submit. 

Once you have applied the required changes to the relevant employees where a Manager is not required. Navigate back to the Mandatory fields page, and re-tick the option. This will ensure all other changes in future are correctly applied and managers are linked accordingly. 

The employees with no manager will then be head of the Org Chart with their corresponding downline.