Well these sort of events seem like the sort of thing building a spy network in a county should increase the likelihood of, even if it's only the +15% or so (depending on intrigue skill).
This event has nothing to do with your character, there is no way your councillors' jobs can affect it. You only get it because you happen to be the liege of a character who was being spied on. The only thing that could possibly increase the frequency of these reports is decisions made by the AI directing the character who reported it, and traits that the reported character has.