Greninja got no knockback from ryu's Down B?

[holymario]

Can someone explain what happened here? I hit him with a fully charged Down Special and he didn't got any knockback.
Or am I just missing something obvious cause I don't play ryu that often :|

 

[buzzard]

I think it's because you hit him while he was still in the air.

 

[Rawbinator]

He slid off the ledge. The stun wears off instantly when that happens