How does knockdown work?

[notpi31419]

How come Falcon's stomp and Fox's uptilt sometimes send the opponent into a knockdown state? Would the opponent have to ASDI down? It seems like against CPUs, Fox's uptilt never knocks the opponent down, but Falcon's stomp often knocks the opponent down, but in matches against people, both attacks often knock the opponent down.

 

[-ACE-]

Downward ASDI is the culprit in those scenarios.