Simple.. The server does not warn you when it is about to restart. This means if you are walking through the middle of a town, and the server restarts, you could be in a very bad position. When you log back into the same server, zombies will be respawned.. When you spawn in, you make "reloading" noise and you are standing out in the open. Zombies then aggro onto you, and you take damage and/or die.
Alternatively, you can join another server, and spawn in right in the middle of hostile players, and you die.
Either way, no good.