I think difficulty is rather irrelevant. I'm playing a french campaign on H/M and for the last 6 turns I've been getting DoWs at a rate of pretty much one per turn from people I don't actually have a border with.

Funnily enough, I'd say this is somehow affected by the number of trading partners you have. I noticed a sharp increase in DoWs after getting myself 4 more "trade for military access" deals...