Guild Wars 2

Druid Longbow #4 is proccing Ancient Seeds.

Ancient seeds states:
Striking a stunned, dazed, knocked down or launched foe summons roots to entangle them.

Longbow #4 states:
Push back your foe with a point-blank shot. The closer they are, the farther is pushes them back.
Knockback for range less than 300: 600
Knockback for range between 300-600:500
Knockback for greaterthan 600: 400

Knockback is not Launch and there are specific differences as detailed in the wiki.

- BUG REPORTED 12/12/2016 -

Ancient Seeds
Anomaly: Although not stated in the trait description, it will trigger on attacks made shortly after a target has been knocked back or pulled.

I agree that Knockback is not Launch but Knockback knocks back. So not a bug, just a shady description.