The Musical Excellence Award is for sidemen
Established in 2000 as "Sidemen", the category "honors those musicians, producers and others who have spent their careers out of the spotlight working with major artists on various parts of their recording and live careers". A separate committee, composed mainly of producers, chooses the inductees. In 2010, the category was renamed to the "Award for Musical Excellence". According to Joel Peresman, the president of the Rock and Roll Hall of Fame Foundation, "This award gives us flexibility to dive into some things and recognize some people who might not ordinarily get recognized."
Is LL Cool J a weird pick for that?