Doddler, I would suspect that it might be the devourer golem, yes. Or, if all of the on-alert plants are at their capacity based on your having neutered them all or them being at a smaller ship cap in general, then the AI is may be choosing to reinforce their home planets with a slightly higher priority than other planets around. I'd have to see a savegame to really know what is causing that specifically here, but whatever it is isn't new based on border aggression (that's a whole other, and very isolated, pocket of code). In pre-border-aggression terms, what you would have seen was just a very highly stocked home planet for some reason. Might be related to a devourer, a dyson sphere camped out at an inopportune location, or something else along those lines, would be my best guess -- but short of a save, I'd be hard-pressed to say for sure.