Initially, from 1990 to 1993, Souter tended to be a conservative-leaning justice, although not as conservative as Antonin Scalia, Clarence Thomas or William Rehnquist. In Souter's first year, Souter and Scalia voted alike close to 85 percent of the time; Souter voted with Kennedy and O'Connor about 97 percent of the time. The symbolic turning point came in two cases in 1992, Planned Parenthood v. Casey, in which the Court reaffirmed the essential holding in Roe v. Wade, and Lee v. Weisman, in which Souter voted against allowing prayer at a high school graduation ceremony. In Planned Parenthood v. Casey, Anthony Kennedy considered overturning Roe and upholding all the restrictions at issue in Casey. Souter considered upholding all the restrictions but still was uneasy about overturning Roe. After consulting with O'Connor, however, the three (who came to be known as the "troika") developed a joint opinion that upheld all the restrictions in the Casey case except for the mandatory notification of a husband while asserting the essential holding of Roe, that a right to an abortion is protected by the Constitution.
After the appointment of Clarence Thomas, Souter moved to the middle. By the late 1990s, Souter began to align himself more with Stephen Breyer and Ruth Bader Ginsburg on rulings, although as of 1995, he sided on more occasions with the more liberal justice, John Paul Stevens, than either Breyer or Ginsburg, both Clinton appointees. O'Connor began to move to the center. On death penalty cases, worker rights cases, criminal rights cases, and other issues, Souter began voting with the liberals in the court. So while appointed by a Republican president and thus expected to be conservative, Souter came to be considered part of the liberal wing of the court.
Roberts is probably on a David Souter career path at this time. What is it that seduces these guys to the anti-Constitution side? Are they simply weak-willed or weak-minded and thus easily swayed?
My guess is that Roberts and other judicial traitors become more and more concerned about their reputations in the eyes of the liberal law school, intellectual elite. Like G. Bush, they want to be liked. That is the only explanation I can think of to make a judge go against his/her own principles. Either that or someone/something is threatening them.