This is linked to an exciting new update!
We're trialling a system where, just like NPCs won't attack you first if you've got double their combat level, you can't attack an NPC that's less than half your level unless it attacks you first.
We were worried about the potential balance implications*, so we've only rolled it out to the various kinds of ducks for now. Wait for the Drake to attack you, then have at it!
Do send your feedback if you have any thoughts!
*We're pretty sure, having done the maths, that this would have the effect of disabling all combat in the game. There'd be a small impact on XP rates.