My enemy's ally is lower my province economy while at peace

[dherve10]

Somehow my enemy's ally is beseiging my province while I'm not at war with him.

[Jekky]

Usually, the only way occupy the province of a person at peace is to attack a mutual enemy who is already occupying it (in which case you can choose to retain the province afterwards without declaring war - but it has a high chance of angering the original owner so much that they will declare war anyway)

[dherve10]

Here's my save file. Hadrigal is sitting in Wagonfort. I'm Brogan Hur. Also, upon further inspection, they seem to appear there after I load a save file.

[Dunadd]

Enemy kingdoms sometimes invade you without declaring war (rarely, but some do) - and there are also some spells that reduce your province's economy that other realms can cast on you (e.g The Iron Barony's 'Acid Rain' spell)