The air horn system was put into place for tsunamis. If those horns went off, people would have likely gone uphill towards the fire, creating even more pandemonium. These was likely 10 minutes maximum from the first fire in Lahaina, to when people were fleeing into the ocean.
Brush fires are common in the islands in the summer, mostly on the old sugar fields that are no longer being used, but always in more remote areas. This recent fire was likely debris or branches flying onto power lines, taking down the lines or simply setting off sparks.
The system in question is the alert sent to islanders cell phones.