They seem to all be fish that live in fairly fast flowing waters, my guess was they are able to use rocks+suction system to hold in place for stuff, they probably don't even really have a concept they are climbing a wall, it could just feel like a particularly intense rapid?
I would say that these are not sharply defined categories, there is more a like a smooth continuum from "fast-flowing stream" to "rapids" to "small waterfall" etc.
And this is a good environment in which evolution could progressively improve rock-holding and climbing ability.
I'd figure it's slightly more complex than that (see details in article) - but I bet you're right about both the evolutionary basis for the behavior pattern, and for having bodies well-adapted to doing it.
