Some say that the occurrence of manslaughter was partly the high priest’s fault. Rashi tells us that a high priest had to pray that no manslaughter might occur. If it did happen, it might be because the high priest did not pray hard enough or work hard enough to prevent it.
Another view is that when a high priest died, it was such a moment of tragedy for the people that all thoughts of vengeance were banished and it was safe for the manslayer to emerge (Maimonides).
JH Hertz believes that the death of the high priest, “like that of a king… represented a definite landmark in the national life” (Pentateuch and Haftorahs, p.722).