I have no idea, I was offline. When I logged in to check on the troops, they were already attacking former ally forces. According to DE I attacked him, which of course I had not done. We made peace by accepting a share map offer.Even though peace was established, our relations updated, the same with the paper where the usual article appeared, markets open again etc.. some of the sea travelling troops kept on fighting. After a while a new article popped up in the paper that DNG attacked me. And war continued.
Other one, had same when he was offline, what I could see, his single arty was at AI range and its could start war.
And to me, coalition member left, cos went inactive. At first hours when he went inactive I had right on the way, I sent in troops to take positions to take over at morning. I logged in and I was at war with that nation.
Will keep updating if something similar will happen.