I've always wondered what the threshold was for setting off all the ships on a planet. I rarely, if ever, see anything that seems to fit into the 'mini battlefield' description. Much more often, I tend to enter a world and immediately all the ships in the world come beelining towards my fleet. Now, admittedly, I do tend to bring big fleets when I come to take a world, because I'd much rather roll through it and be done rather than lose all my ships and have to come back. So maybe the problem is that I'm bringing too much and triggering some sort of 'free everything' response.
Now that I think of it, if the intended behavior is for the human team to bring 50 or so ships and fight out the battles one guard post at a time, the game isn't doing a very good job of conveying that idea. If you bring a fleet blob (which, really, who wouldn't bring it if you have it available) and you see that the AI's response is to send everything on the planet at your fleet, how could you be expected to come to the conclusion "ah, I should bring a fraction of the ships next time, that will work better"? The obvious assumption is that you bring fewer ships, and the AI still sends everything at you, and you die. So we'll keep blobbing and the 'mini battlefields' will keep falling apart because we're triggering the AI's 'OMG KILL IT' response.
I'm not sure what the solution to this problem is, but that's probably why I'm not developing games.