roberts was chosen by bush 2, after rehnquist died.
there is no seniority/pecking order...the chief justice is chosen by POTUS at the pleasure of POTUS, when the opening presents itself, I believe.
there is a senate procedure to confirm, as I recall.
...but, I may be all wrong here.
Rehnquist, already an associate, was promoted to Chief Justice.
Roberts was appointed Chief Justice.