Indeed, I suspect that the activation time is intentionally long so that the scenarios feel more "organic" - if they always triggered right after you drop in / approach the facility, it would be even more obviously scripted.Ah. I've visited several, obviously I wasn't waiting long enough.
Incidentally, perhaps related to some state change, I noticed today that the megaship right next to Davies has started generating defense scenarios. The raiders aren't averse to bringing wanted Pythons with half million credit bounties, either.