GammaKnight
Smash Rookie
Actually, with further testing, I've discovered that the Mario characters can fall into the box trick by themselves. All they have to do is fall into that exact spot without teching and they're stuck until somebody grabs them, hence why it works with the Space Animals' down throw.I don't think that all characters could do this this glitch I think it has to do with the Fox's and Falco's style of down throw
this is how I believe this works(along with a few other people who I've discussed this with)
After Falco throws Luigi down, he is forced to the ground in a stunned state briefly. Also, Luigi's hitbox comes in contact/overlaps with the stage for a certain amount of time. During this moment, Luigi cannot move until the attack is complete (hence why all of Falco's lasers connect). Over time, though, Luigi escapes this stunned state, and his hit box no longer overlaps with the stage.
Now as to why they get stuck, my theory is that the way the programmers overlooked an "opening" in the ground, and that the size of the Mario characters' hitbox/sprite when they fall down onto the floor allows for access through this opening, only that they can't get out of it because they can't fit through the opening when they're standing up since their sprite/hitbox is larger than when they're laying on the floor.