From looking at these vids, my first theory would be that with a ceiling, since you can't go through them (and sometimes even if you can? o.0) the game moves you and the invisible floor to adjust for moving along the ceiling. On some stages, this special case occurs near an edge, and when it occurs near an edge with a lip the opponent is stage spiked by going up directly under the lip. Kirby is fine, as he is usually clear of the edge, not to mention his uthrow doesn't give him knockback.
Hyrule is weird, I have no idea why the invisible floor moved under the platform. ... But anyways, this can help guide us in our testing by doing it only on stages like that first. Testing this on FD/BF/etc. is pointless.
EDIT: ... unless maybe moves that give velocity without knockback might also help this? I remember MK and G&W doing something similar to this using G&W's up+B in the middle of MK's Uthrow and it moved the invisible wall up. Maybe Mario/Squirtle could give us a push?