There was a post about this in the Conan forums a while back: Doom spends vs. just doing normal GM things?.
Basically, I am of the opinion that if a player spends threat to do something, generally a threat spend will be a response to that (the STA GM Guide portion I quote has an example of a character using threat to climb a crumbling cliff, the GM then uses threat to make the cliff start crumbling even more, or the crumbles alerting guards).
Obviously a GM starts with threat, so that can’t be spent ‘in response’ to a player threat spend, but the other reply to that thread talks about using threat to up the difficulty of rolls in previously planned situations (which I guess is similar to your pistols > disruptors example).
Rule changes and other thoughts also has some discussion about this topic.