To give you an idea of positioning when this happened (at least in the Jigglypuff scenario), I was recovering off the right side of the stage and DIed till I hit the UpB sweetspot range. Jiggly was falling down above me with a Nair, and just as she was about to hit me, I UpBed. I proceeded to UpB and grab the righthand ledge, while Jiggly went flying off into the distance to the right and inevitably KOed at 0%.
Now Eighteenspikes: it was not a stage spike, that would involve Jiggly hitting the lip of the stage as I upBed. This would cause to fly to the left instead of the right, not what I'm talking about.
Also, one thing I'm very certain of, is that the only reasont his KOed Jiggly is because she just didn't see it coming and DIed the wrong way. I sincerely doubt this will KO at 0%, even if it is on Yoshi's.
Also Kiru: I don't know if it was a reverse UpB, it might have been can't remember. I know the Jiggly situation was on teams, probably adding to the extra confusion of the situation. And against Nick it was on BF recovering towards the lefthand ledge of the stage.
All in All, I have no idea what specifically caused them to go flying backwards, but I just want to clarify that it is not a stage spike as they weren't close enough to the stages to even hit it. Plus, stage spike KO's tend to result with deaths off the bottom of the stage (hence 'spike'), in Jiggly's case it KOed her off the side.
The best lead we have so far might be Dogy's suggestion that it was the first frame of the hitbox, and only the first frame. IIRC, I believe only one hit made contact both times, so that's probably what we are going to have to assume right now. As for why it sent them in the opposite direction, any ideas?