Why devastation comes back in certain provinces for no reason?

[byebytoad]

I'm playing as Milan, and in certain provinces devastation keeps coming back even though those provinces have not been invaded (or even remotely crossed by foreign -whether friendly or enemy- troops) .

I do some development on those to bring the devastation down, and see the devastation go to 0.
Then after a while I notice devastation is back.
Anybody knows why?

 

[Bouchart]

Blockades cause devastation.

 

[Mr. G]

And piracy.

 

[byebytoad]

Thanks I didn't know that