Morale influences

Depending on the provinces your province borders, that will affect morale development(There are other factors too). Neighbors at war negatively impacts morale development while provinces closer to your capitol or near allies, or near provinces you own will affect it in a positive way.

Yes it writes to me decrease of morale in category "enemy neighbours" but it also writes to me much larger decrease of morale under the category "neighbour". It's around - 20 for example when around it are only my own provinces.

That's a case of unhappy provinces causing nearby provinces to also lean in that direction