samurai of the pale curtain and mass removal

[Ender]

What is the officially reason why if you play something like wrath of god with samurai of the pale curtain in play all creatures are removed? Is it last known information or something else?

Also, is it the same answer/reason if Nevinyrral's Disk is popped with compost in play and a bunch of creatures?

[Matt]

Yes, it is the same case.

Cards see each other go to the graveyard.

410.10. Trigger events that involve objects changing zones are called "zone-change triggers." Many abilities with zone-change triggers attempt to do something to that object after it changes zones. During resolution, these abilities look for the object in the zone that it moved to. If the object is unable to be found in the zone it went to, the part of the ability attempting to do something to the object will fail to do anything. The ability could be unable to find the object because the object never entered the specified zone, because it left the zone before the ability resolved, or because it is in a zone that is hidden from a player, such as a library or an opponent's hand. (This rule applies even if the object leaves the zone and returns again before the ability resolves.) The most common types of zone-change triggers are comes-into-play triggers and leaves-play triggers.

[Jacob Orlove]

I think Matt meant to quote rule 410.10d

Abilities that trigger on one or more permanents leaving play, or on a player losing control of a permanent, must be treated specially because the permanent with the ability may no longer be in play after the event. The game has to "look back in time" to determine what triggered. Each time an event removes from play or changes who controls one or more permanents, all the permanents in play just before the event (with continuous effects that existed at that time) are checked for trigger events that match what just left play or changed control.
Example: Two creatures are in play along with an artifact that has the ability "Whenever a creature is put into a graveyard from play, you gain 1 life." Someone plays a spell that destroys all artifacts, creatures, and enchantments. The artifact's ability triggers twice, even though the artifact goes to its owner's graveyard at the same time as the creatures.

[epeeguy]

What is the officially reason why if you play something like wrath of god with samurai of the pale curtain in play all creatures are removed? Is it last known information or something else?

Also, is it the same answer/reason if Nevinyrral's Disk is popped with compost in play and a bunch of creatures?

Actually, both Matt and Jacob are a little off with their explanations, and the reason behind these two are different.  While it is true that triggered abilities go back and look at the game state just before the triggered ability would trigger, static abilities are handled a bit differently.  In the case of Samurai of the Pale Curtain's ability, which is a static ability, the ability looks at an event that is going to happen (a permanent going to the graveyard).  If that event were to happen, Samurai of the Pale Curtain's ability "changes" it to something different (the permanent leaves play instead), modifying the event that is going to happen.  So, it looks a little like this:

Event to happen:  Permanents go to graveyard (as a result of Wrath of God or Armageddon)
Replacement:  If permanents go to graveyard, they are removed from the game instead.
Event that actually happens:  Permanents are removed from game.

Because Samurai of the Pale Curtain's ability is functioning at the point the event would occur, it will modify the actual event that happens.  This is why Samurai of the Pale Curtain removes itself from the game, as well as any other permanent that would normally go to the graveyard at the same time the Samurai does.

The applicable rules are 217.1d, 419.4, and 419.5.

[rvs]

Epeeguy is correct. This was also in the CHK FAQ IIRC.