Playing as Thrace m/m and I started the game by making alliances with the Scythian and Dacian barbarians so I could concentrate on taking the Greek peninsula from it's corrupt and decadent populace. These alliances were made in the 2nd and 3rd turns respectively, so they were probably amongst the earliest alliances in the game.
I declared war on Macedon and took Thessaloniki and Bylazora fairly quickly and started to produce falxmen and pikemen whilst building up a fleet in other cities.
The Macedonians must have been busy elsewhere as they sent no more troops to reconquer their lost territories, so nothing happened for about 8 turns.
Then a Macedonian bireme appeared off the coast of Thessaloniki so I attacked it with my 2-star admiral stack - as you do. A message appeared saying that this would be construed as an act of war against a neutral country, and was I sure I wanted to do this? I double-checked the ship and it was definitely Macedonian, so I shook my head in wonder and clicked the 'YES' button to declare war.
After the battle I received messages saying that war had broken out and that my alliances with Dacia and Scythia were in tatters.

What the hell happened there?

Has anyone else had a ceasefire happen when they didn't know it? I never even received a pop-up saying that hostilities had stopped or anything.

And why have Dacia and Scythia broken their alliance with me? Now there isn't even an option to form an alliance when I go and speak to them.