Quote Originally Posted by The Grand Inquisitor
I think inquisitors increase Zeal and reduce heresy, until the zeal reaches a critical point (not sure what this value is), and then go on a burning rampage, which drastically reduces zeal (and happiness).

I also tend to keep inquisitors on foreign soil and go after low piety generals in packs.
A rather appropriate discussion for you, Grand Inquisitor

I believe that the threshold is turn based rather than zeal based. I've had inquisitions begin in regions at 80% zeal that started off high zeal (50-60%) while other times they've begun at just 50% in regions that originally were low zeal.

I do recall somebody mentioning the tip that picking up your inquisitor & placing them down again restarts the whole process and can help avoid inquisitions although I haven't yet tried that since I'm currently going through a no-crusades mood - my troops fight for their king, for glory, plunder and pillage. And the king always provides dancing girls and an amusing retinue of fools and jesters.