What is the term for the "process of hiring external HR professionals to do the HR work that was previously done internally"?

HR Outsourcing

Explanation:

Human Resources (HR) outsourcing is a popular business practice where firms in order to allow its HR employees concentrate on the core HR functions of the organization, engages consultants who are experts in HR matters to manage some of the HR tasks of the organization.

The need to sub-contract human resources functions always become necessary when HR tasks arise that may be considered complex, time consuming and capable of shifting management focus from the core HR functions