Yep, that's something that happens frequently in every scenario that i've played; once friendly countries, they seem to start throwing insults to you, forcing you to take immediate measures against imminent invasion. Sometimes it is possible to improve relations and revert the situation, but you'll have to wait sometime since the diplomatic relations are suspended for several turns after throwing an insult.

The funny thing is when smaller countries/empires decide to invade you: In one of the Chinese scenarios, i had a very small empire (3 provinces all in all and a non competent army force) theatening my 59-province empire. Not sure if they felt threatened by me having an army force near them (i didn't even think of doing them harm, i was just being careful of the other nearby empires), but they sent most of their army across my borders completely out of the blue (well, they send the usual insults before the invasion..).

Anyway, as i see it, AoC 2 has less ties to reality & logic and it's more like a board game where everyone tries to backstab the others, either by being fast raising an army or by exploiting them by use of diplomacy, until the point where someone crosses the borders. All in all a fun game, not one that will make you plan very carefully your next move though.