Decaying Soil
At the beginning of your upkeep, exile a card from your graveyard. Threshold — As long as there are seven or more cards in your graveyard, this enchantment has "Whenever a nontoken creature is put into your graveyard from the battlefield, you may pay {1}. If you do, return that card to your hand."
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Official rulings (2) · Scryfall
The ability in Threshold will trigger only if Threshold was met before the creature goes to the graveyard. So the one that causes Threshold to be met will not trigger the ability.
Multiple creatures going to the graveyard at the same time all end up there at once, so either all of them or none of them will cause the trigger to go off, depending on whether or not Threshold was met prior to the creatures going to the graveyard.