The main driver is not HR wanting a back-up's or the company mollifying investors but a reduction in the power of mid-level managers.

Before, headcount was basically owned by the manager, and s/he filled positions to maintain their headcount.

Now, headcount is always provisional, and typically discouraged. Managers protect themselves by reducing their headcount through attrition. So actually hiring takes sustained effort by a strong manager. The bar is very high because businesses know that hiring one person costs $millions over the typical employee lifecycle.