I've noticed, or at least I think, I get fumbles whenever I do something that applies an status efect (even when hitting with weapons).
In my new playthrough I've made a Battlemage and a Cleric, and I'm seeing a lot of Fumbles with the battlemage, even though I get no penalty for low intelligence/strength. I was worried about it being a big penalty for having low atributes or something, but if it's only a status effect thing, I'm not that worried.
Thanks for the answer.