When it was announced last week that Philadelphia Orchestra principal violist Roberto Diaz would be leaving his position to head the prestigious Curtis Institute of Music, the near-universal reaction from the rest of the orchestra world could best be described as, “Huh?!” The fact is, musicians holding plum positions in the world’s top orchestras almost never quit mid-career, and many have to be pried from their chairs when they can no longer do the job. Diaz, a legitimate star in the (admittedly obscure) world of the viola, now holds the distinction of having walked away from not one, but two highly regarded orchestras, and he couldn’t be happier.