I just don't see why he would be reinforcing his homeworld, it's not on alert or anything right now. Here's a save I was going to use to report a different issue (Teleport Raiders after destroying a command station would sometimes become completely inert and not move). It's at a point where the AI is sending core raiders at me. All the on-alert worlds have no more than 400-500 ships, but the AI is constantly reinforcing his homeworld. While I don't have sight of the homeworld in the save, a quick suiciding to reveal the map shows that the AI homeworld, Yed Posterior, has 3408 AI ships, and most of the core worlds have 1500-2500 ships as well. It's worth mentioning that 700 of those are permamines, its interesting that mines count towards the border aggression counter!
So I looked into this, by the way -- the first thing I notice upon loading this savegame is that there are huge clusters of their planets on alert, including their homeworld. So that solves the mystery right there -- they are reinforcing there because they are on alert there. You can easily tell which planets are on alert by seeing if their borders (or, for the unexplored planets, their whole graphic) shows as yellow instead of white. If that is hard to see (color blindness or just monitor contrast or whatever), then you can also hover over planets to see which are on alert and which are not.
That said, of course the second question is why those planets were on alert. When I cheat and enable visibility, I see the devourer golem is on a planet right next to the AI homeworld, and is duking it out with a bunch of core ships. Looks like he must have been hanging around in that corner of the galaxy for a while, and so that's what is resulting in the strangeness you were seeing.
I'm actually pretty pleased with that sort of functionality at present, because normally the devourer golem just makes things so much easier -- now there is more of a consequence to him, as well, which seems like it will make him more attractive for more people to use.