Ok, so you want to get trees that fly.
Let's see what you need for that:
OK, the first thing you need for flying trees is a way to generate uplift.
Why would the trees evolve uplift?
Certainly not in order to fly: With only little uplift, you cannot fly, but you first need to generate a little uplift in order to then evolve to uplift strong enough to fly.
So what would trees need that uplift for? Well, uplift basically means pushing air down. Now when stationary, pushing air down means that fresh air from above replaces it. So maybe the trees evolved in a low-CO2 environment, and the consumption of CO2 by the trees mean that the concentration of it went down. Therefore it was an advantage for the trees to ventilate fresh air with more CO2 into the forest.
How would the trees evolve uplift?
I think the most logical step would be to grow moveable branches that can act similar to wings, with leaves evolved to have a similar behaviour to the feathers of a bird's wing. The hard part would be to explain why they evolve the movable joints, as trees don't normally have that. But maybe there are many relatively fast moving, dense clouds, and being able to quickly move branches vastly improves the amount of light the tree can harvest.
Ability to remove the roots from the ground
Frankly, I have no idea why or how this would evolve. But since you are talking about crawling trees, I'll just assume that at the time when trees evolved flight, this had already evolved.
Ability to leave the ground
Bow that the basic properties are evolved, the next step is why trees would actually evolve to leave the ground. The how is easy: The uplift just gets strong enough to lift the tree when not rooted. The why could be that it is an advantage for the tree to shortly hover when animals start to eat from its bark. Since the animals which do that usually can't fly, by hovering the tree could get away from that animal.
Ability to full-fledged flight
Well, from that point on it is a quite direct path to full-fledged flight. Obviously it's an advantage to be able to hover for a longer time; also a better control of the hovering is advantageous. And with that comes the ability to actually change position; the advantage of that ability should be obvious.