okay that is good to know, but i didn't saw a evil biome at the embark screen so i dont know if i am in a evil biome. also this happand in year 3 but the corpses where fine in the 1st and 2nd year.
i do sometime have a intruder that is a necromancer, but they never got near my refuse pile.
this started happening when my dwarfs killed 1 zombie and brought his corpse to the refuse pile.
You probably have a necromancer. Necromancers can arrive stealthily, like ambushers, and sneak into your fort unseen. They can also raise the dead without breaking cover, and they seem to be much more difficult to detect than goblins or kobolds. I've had soldiers literally surrounding a necromancer without detecting it - I could see where the necromancer was by the door it held open, and my soldiers were in the 8 tiles around that one, yet it remained unseen.
Check your combat logs. If there is a line that says that a necromancer is fighting, you probably have one. Also, the first indication of a new zombie's life will be a "shudders and begins to move" line in the combat log if there is a necromancer around. If there's no such lines, you have an undead biome. Also, if all of the bodies in an area are standing up simultaneously, that's a sure indicator of a necromancer, as undead biomes raise a corpse a short time after it died rather than when something else occurs.