I think that I have some idea why and how this happened. I could be totally wrong and way off base, but here we go.
We have seen several techniques that all stemmed from the same basic mechanism: Snake's sliding usmash, glide tossing, Diddy's rising up-b, and probably more. In each case, the character does something (a dash attack for Snake, a roll for the glide toss, and a jump for Diddy), then immediately cancels that with another move (a usmash for Snake, a toss, or an up-b for Diddy), but they maintain the momentum of the first move.
In short, the bug appears to be that you can cancel a move (or just some sorts of moves) if you input another move right afterwards, but it doesn't cancel the momentum of the first move. In this case, I would bet that you rolled, then immediately spot dodged. To reproduce this, I would hold shield, then roll by pushing the control stick down (a bit) and to the side, then rotate it down as fast as possible.
This looks like it could be big. If this pattern continues, we could potentially see a lot of move-canceling tricks. I've tried out a few with Snake, but to no avail. For an example, I've tried to cancel his dash attack into a grab or a shield, but neither has worked so far. Likewise, I've tried to cancel a roll into a grab (which is the exact same mechanism as glide tossing, except without an item in your hand), and couldn't get that to work. I haven't found any real pattern in which moves can be canceled, and what they can be canceled with.
Anyway, this is a nice find, and it could be really big indeed if it is significantly superior to a roll. It looks like it has much less lag on the end, so it probably will be.