Emerikol
Legend
Well, there's plot, and then there's plot. And there's levels of preordination.
In my Deadlands campaign, the PCs dropped the ball in a major way, and left a McGuffin they knew was powerful and important in the open, having nearly painted a sign on it telling the bad guys where it was. Technically, in theory, the PCs could have then gotten in the way of the next steps. But they didn't know to do so. The Cthuloid horror now wandering around in Missouri was pretty much a foregone conclusion after they failed to show common sense.
Well you have a given set of bad guy npcs that reacted to something the PCs did. That fits fine with what I am saying. In fact it's exactly what I'm talking about. I do think as much as you can it's better to at least have an overall understanding of your npcs in advance so these things aren't as contrived. And by no means am I saying yours was. Just saying it works better if you know things about your npcs. And my use of the term npcs of course includes "monsters" etc...
For me at least, I like to develop a lot of "plots" that are happening behind the scenes between the various npcs. What becomes a real group plot is then up to them.