They all trigger simultaneously because they commit/move simultaneously. That is the triggering event.
You're conflating a trigger condition being met and an ability being triggered when these aren't actually the same thing.
A trigger condition causes something to BE triggered. However, if there are multiple simultaneous triggered abilities from one event, then they are triggered sequentially. If one of those triggered abilities becomes invalid because the board state has changed (i.e. the card is no longer in play) it CANNOT be triggered, even if it was in play when the condition was met .
A trigger condition will occur, but you only check the validity of an interrupt or reaction when you attempt to trigger it.
To give another example, if I have a card in play with Stracken's Cunning attached, and it is killed by an enemy attack. I draw three cards. One of those three is Elysian Assault Team. Even though that card wasn't in my hand WHEN the trigger occurred (friendly unit being destroyed) it IS valid for me to trigger the reaction, as we are STILL in the reaction window.