First, HMAC comes from academic researchers Bellare, Rogaway, and Krawczyk. NSA never touched HMAC, the standard is exactly as defined by the academic researchers.
The rationale for the choices of ipad and opad are explained in their paper Keying Hash Functions for Message Authentication on page 14, but unless you are a cryptographer, you might have difficulty understanding it.
In a nut-shell, there is nothing special about those particular values. Any constants would have worked.