1 dies of a heart attack in his sleep, but isn't found for over a month, because he locked himself away to work on a big project; which s/he is prone to do.
2 got bored and went of vacation never to return in a continent far away.
1 made a deal with the devil to gain his/her power and the devil came to collect. He sacrificed himself instead of 1000s of deaths and souls of his nation. But he deserves it because he cheated to get power.
1 was asked by a god to help defend the world from something from beyond.
1 gets bored of living and says "screw it, i'm out".
1 one wants to live forever, and goes the lich route but doesn't tell anyone. Some random street urchin drops the secret urn that was a phylactery, mean while the lich trips down some stairs, but didn't prepare a fly spell.
1 one is poisoned, but didn't prepare remove disease, or never learned it, dying of his wounds (it could be multiple poisons.
1 is assassinated, easily done, because the mage had hubris and never locked his door, or had guards.