In my current game, I was lining up to attack the thalians.  They had 2 military resources, and both of which I parked a stack of constructors and battleships next too.
Meanwhile I was manuveuring the rest of my ships to prepare for war and I noticed that one of the starbases was suddenly vacant.  I looked to see if someone had destroyed it, and no-one was at war with them.  I haven't got the pirate event, nor have I seen any.  So what happened to the starbase ? 
I've seen this a few times, but this time I'm certain that it wasn't attacked.  So they must have demolished it.  I think I'd have more respect for the AI if rather than doing this, it sold it to me or another AI.  Which makes me wonder if it's a bug.  Has anyone else seen this ?