Yep ... All thanks to recent changes in the 'Elite AI' software!

Apparently you have low popularity because you have attacked without provocation a few AI managed countries ... Should you continue on your aggressive path, they'll declare war on you!

This happened to me in the last game, where I had Turkey in the 39 map and was playing rather aggressively ... At one point I was at war with 8 countries- 6 of them AI managed and they all declared war on me! ... I had 2 nights of nearly sleepless nights defending myself, but I managed to keep my winning streak ...

When looking at the diplomacy menu you can select the details of each country. This will list your popularity with that country.

My observation is that when your popularity reaches 10% a trade embargo is declared. When your popularity reaches 5% war is declared.

A trade embargo from a bordering nation will likely result in a war declaration in short order. Likewise if you conquer one nation and suddenly have a border with a new nation that has already embargoed you, then the war declaration seems to be a self defense measure.

There are likely other triggers for these actions as well, but this is my generic observation.