After the collapse of the eruption column, when it's roaring down the volcano's flanks and incinerating/burying everything in its path, it is driven by the local topography.

But before that collapse - especially toward the end, when it's getting seriously unstable - the wind can have huge effects on the direction in which it "fall down".

And the wind (at altitude) pretty much controls which way the ash fall goes. That's technically not pyroclastic flows - but often just as deadly for a major eruption.

EDIT: Add Wikipedia's account of the eruption - https://en.wikipedia.org/wiki/Eruption_of_Mount_Vesuvius_in_...

- And note that descriptive terms for volcanic eruptions (Plinean, Pelean, Vesuvian, etc.) are subject to "it's all constantly-changing shades of gray" caveats.

The wind _positions_ the ash cloud and then the terrain channels it once it falls. I suppose I'd been thinking of a Mount St Helens style flow where the mountain gave way.