You have met what i'm considering as the worst bug of this game.
Nobody can exactly tell what happens. I've heard plenty of rumors on it, such as difference of chipset between user comp and server who compute the turn or a bug between two version of the game/mods ?!
But some other people have met this bug in solo too, so ...
However it seems more frequent in MP than SP.
Hopefully the next time you meet it, it's going to be in your favor
