I suspect you hadn't set Ansbach or Württemberg as your rivals, it can halve the discovery risk and while changing a rival may cost diplomatic power, setting it in the first place doesn't. The tool tip is a bit misleading.
So are you going south to conquer those flag-stealing bastards again?
Hmm... I didn't know this. The full list of what rivalry does is a bit unbeknown to me. I know it has benefits when embargoing and such. And Württemberg should definitely be a rival of Ulm. It's a bit late to make Ansbach a rival at this point. But considering that I need alliances with Bavaria and The Palatinate for now, I am not just going to make rivals of everyone. Diplomacy is essence to a strategy for an OPM.