The final conflict between the Rhoynar and the Valyrian Freehold is named the Second Spice War. Why this war started, we are not told but my resistance to tax enforcement theory (on spices for this instance) could be a genuine reason or just a pretext to do something Volantis had wanted to do to neighboring Sarhoy for a while perhaps. The initial belligerent is as far as we can tell is likely Volantis, a colony initially intended as a military post but became a wealthy trading port. The Volantenes with the assistance of "three dragonlords" not only sacked it, sold the survivors into slavery but also had "their proud pink city put to the torch" then salted the smoking ruins. TWoIaF does not say it was burned by dragonfire but sees fit to mention the assistance of dragonlords and then later in this war 3 dragons do take the field against Prince Garin's unified Rhoynar forces so I'm not convinced dragons weren't used against Sarhoy in some way, even for something as simple as burning through opposing naval lines. Now, why may they have want to do a sack, salt, and burn on a profitable port city instead of just conquering it and continue to profit there? Possible combo of: a) Likely not enough manpower to hold it even though they enslaved and sold the entire population b) Because of the physical of position of Sarhoy would be strategically useful for staging a military operation against Volantis cannot allow anybody else a chance to hold Sarhoy either. c) Scare tactic against the other Rhoynar city states not cooperating with what Volantis and other colonies wanted So absolute and intentionally lasting destruction.