While Scalia's death is unusual in recent history, that's not the case over the full length of American history. An analysis by Marquette University's legal blog in 2012 said that 38 of the 57 Supreme Court justices who served between the nation's founding and 1900 died while in office, while 39 of the 46 justices who have served since 1900 left in retirement. "For the half century between 1955 and 2005, there was not a single death of a sitting Supreme Court justice," the blog notes.
Why was it so much more common for justices to die in office during the Court’s earlier history?A shorter life span for the justices is clearly part of the answer. Seventeen of the first 38 justices to die while in office died prior to their 70th birthday, and four of these, Wilson (56), Iredell (48), Trimble (52), and Barbour (58), died before reaching the age of 60. In contrast, the six justices who have retired since 1990—Souter (age 69), Stevens (90), O’Connor (75), Blackmun (85), White (76), and Marshall (83)—had either reached, or were approaching, their 70th birthdays at the time they stepped down.Stricter pension eligibility requirements may also have been a factor in the reluctance of earlier justices to resign. For most of the 19th century, Supreme Court justices were eligible for a retirement pension only if they were 70 years old and had served on the Court for more than 10 years.
It is also interesting to note that far more justices have died when a member of the opposite party holds the White House. A 2010 study in the journal "Demography"statistically analyzing the retirement and deaths of Supreme Court justices from 1789 to 2006 found that:
[P]olitical climate effects on death in office are consistent with the politicized departure hypothesis. When the incumbent president is of a different party than the president who appointed the justice, then the justice’s death-in-office odds are about tripled, compared with when the appointing president and the incumbent president are members of the same party.
The finding makes intuitive sense, given that justices do not want to give a president of the opposite party an opportunity to tilt the balance of the court. The study also contained this helpful chart looking at the average age of retirement and death for Supreme Court justices.