Mistwell
Crusty Old Meatwad
To me that means the target can only move itself if there's something to grab on to. There's nothing at all about external forces e.g. a breeze or someone giving the person a push. Same as a paralyzed target being thrown in a river: the target can't move itself but the river can carry it along just like any other bit of debris.
Also, if a person is already moving when levitated would you have that mean the person then stops in place? I sure wouldn't; the target wuld keep drifting at the same speed in the direction it was already moving until-unless external forces dictated otherwise or it got out of spell range.
This whole "breeze" thing is silly. It's 10 minutes long. And no they do not keep drifting it's not anti-gravity creating a void. There is nothing in the text of this spell which says anything about externally being moved more easily by wind or momentum.