Rulings

The ability doesn’t care why a card goes to your opponent’s graveyard — only that it does. The ability triggers when a card is put into your opponent’s graveyard from the stack (a spell resolves, fails to resolve, or is countered), from the battlefield (a permanent is destroyed or sacrificed), from the player’s hand (a card is discarded), from the player’s library (from a Millstone-like effect), or from any other zone.
This ability triggers only on cards, so it won’t trigger when a token is put into your opponent’s graveyard.