Leeward (western) side is dry in that it sees little rainfall.
Recent exceedingly low-humidity has been unusual.
A high pressure system to the north was gently rotating clockwise and the full-blown hurricane well to the south was nevertheless rotating counterclockwise, turbocharging the normal wind pattern (generally west as mentioned) for the Hawaiian Islands squeezed in between these two separate weather events.
A small earlier wildfire of dry wild grass somewhat up the mountain slope was said earlier in the day to have been contained 100 percent, causing some local residents to breathe easy and perhaps lower their guard. But something went ***spark*** a little farther south above town, then screaming straight line winds (upwards of 80mph) seems to have taken that initial flame and created a literal firestorm almost instantaneously.
The irony is the other side of the island gets around 300 inches a year
Look at the Google satellite view of the town. Go to street view and look at all that wild grass north of the town that should have been tilled or mowed. Poor land management, typical for democRATS.