It sounds like in this specific case it is none of the normal things. I suspectitcould be overflow reinforcements if there are fewplanets actually on alert. Let's say you captures 12 planets but by a quirk of the map only 3 are now on wrt. That means each ai now has 12 reinforcement points to spend every cycle, and only 3 best places to spans it on. Therefore each of those 3 gets one reinforcement point every time, and then those other 9 get put... Elsewhere. In your case, not that I've seen the save, it could be that a lot of them have been stacking on this one planet for a while.
Usually if you've taken 12 planets, often you'd have 24 or 32 or more planets on alert (VERY much dependent on map type). Which is actually to your advantage, because as long as all 24-30 planets are no home planets of the ai, you wind up with reinforcements being so diffuse that you have a pretty easy time by comparison.
It's basically a matter of how much you're telegraphing your intentions, versus how much you're distracting the ai with irrelevant alerts. This is one reason the snake maps can be harder than you'd think (despite how easy it is for you to defend on them).