Volcanoes are toxic in nature, and what better Pokémon to eat up those toxins than Weezing? While Weezing is already found on the volcanic island of Cinnabar, maybe Alola's volcanoes are much more toxic in the deeper parts of the mountainous volcanoes? Since Weezing is typically explosive by nature, the Pokémon had to adapt to fire, and as a result took on a Poison/Fire dual-typing.
The image above was retrieved from deviantART user Phatmon. I must say, it's exactly what I expected from a Fire/Poison type Weezing. While it was originally designed as a Mega Evolution, maybe their design will be on the money for a future Alolan Form Weezing!