I don't think wind direction matters while sneaking. It's not that complex of a system. Each creature has a different "detection radius", which I suppose you could imagine includes smell, but it's just a simple cone that can be narrow or very wide depending on the creature. Your sneak skill shrinks the size of this cone, that's it. If you're in the cone, you're spotted. If not, you're good.

The only other factor is light, which is what the bar indicates. If it's bright, your sneak skill is significantly reduced. If it's dark, it's increased.

Basically the trolls turn because their pattern is to wander around in every direction, and when they do, you end up in their detection cone. That's all there is to it.