Its because they trashed it with Catapult.
When you trash with Catapult, up to two attacks may happen based on what was trashed:
- If the card cost 3 or more (e.g. Flag Bearer, which costs 4) your opponents gain a Curse
- If the card was a Treasure (e.g. Copper), opponents discard down to 3
These criteria are checked independantly. If the card fulfils both (e.g. Silver), both attacks happen.