According to CNN, Supreme Court Justice David Souter will retire in June after 18 years on the court. Here is his natal chart:
At the time the news broke (7:59 PM PDT), transiting Uranus (newly entered Pisces) at 0°19' Pisces was opposite his Sun at 0°17' Virgo within 2' - so (to put it simply), he's ready for a change, freedom, to move on to a new life stage, to have time to explore or whatever. Mars was in partile conjunction with his Jupiter. The Moon (again, this is for the time the news broke) was 1' from square his rising natal Saturn.
All of thei mportant Secondary Progressions involve the Sun. His progressed Sun is leaving a (still partile) trine to Pluto, so he's been thinking about this change for a while; and it's moving toward a square to Mars. But perhaps the best indicator of the time is that progressed Venus is 0°23' Sagittarius, only minutes past the square to natal Sun.
People leave the court for many reasons. I think Justice Souter, at 70, is leaving to enjoy himself.