I've had the same (And similar) situations happen to me in a few games.
A friendly race that I have nothing against but happen to pass by with a large fleet on route to an enemy of mine who is not an ally of theirs tells me they're not stupid, and declare war. (Which almost always IS stupid, because it's only a small race, in my experience.)
However I've also observed the flip of the coin, instead of super-paranoia, some races who are just too stupid to protect their planets, even when I take multiple turns to assemble a fleet. Hell, on normal I once stacked up an invasion fleet some 3 squares away froma a Korx planet fully stocked with ships. (We were at war.) Now, they could have defended the planet, had they attacked before I stacked up the fleet, but do you know what they did? The ships actually took off, but rather than attacking me, started en-route towards one of my other worlds, completely ignoring the invasion fleet. Now that was...short-sighted. Not only did I then take the planet early, but by the time their ships arrived, I had a fleet waiting to counter them.
The AI, as it is, is good. Better than the AI of many other games. But nothing is perfect, and it can always use improvements when the devs have time.

(Which I hope they will.)