I know this is more than likely spitballing and grasping at straws, but a fun little theory I cooked up that ties everything we know so far into a Pumpkin Lord's Cursed Halloween storyline:
The pumpkins in Production Central (and probably) New York, the overgrowth and roots in Central Park and Hollywood are all the evergrowing influence of the Pumpkin Lord, with that also tying back to the witches in San Francisco, summoning him with the cauldron and a ritual, just as they did in Wicked Growth last year.
One of the ingredients put in said cauldron for his summoning? A specially-made potion that comes from Prohibition Witches, who also like to ship their twisted lil drinks to a Dead Man's Wharf. Another one of the ingredients for the Pumpkin Lord's summoning? The blood of a Chupacabra victim.
Why did the meteor mutate all the plants in SoX and (presumably) Descendants? Struck down directly on a more powerful spot of the Pumpkin Lord's roots/growth, which is what causes the spores to release from it and mutate all of the plant-life.
For Bugs, I'm going off the theory that its a new type of extermination device that ends up mutating the bugs rather than killing them: One of the factors of said new devices comes from a remnant or seed of the Pumpkin Lord.
And finally, an ultimate retcon, the reason Robert L. Strickland went mad in the first place and ran such a brutal prison: He is one of, if not the
only survivor, of the Pumpkin Lord.
Once again, I know this is most likely wrong and more just a crazy theory coming from the mind of a big lore fan, but it's still fun to think about!