Does Warwick's 'when I attack' effect trigger when he is defending and an opponent unit moves in to start a showdown?

Ruling: No, 'when I attack' effects do not trigger when defending. Attack triggers only activate when their source is given the attacker designation, which does not happen when your units are defending and not applying contested status.

Nuances:

• The defending player's units do not receive attacker designation
• Units that are not applying contested status cannot be the attacker