They could've seen that your neighbor had more attractive systems?

Well, it was specifically a precipitous drop in food production that was causing the loss. After removing the pill plants, things more-or-less stabilized. 

But like I said, those planets were nothing new. I had enough food to go around. It's like some major empire-wide change occurred, similar to what might be experienced after a law was abolished. Just odd.

Could be populations growing bigger than the food production could handle, an event effect could have ended that turn, though I cannot think of one that could do it.

It could be a combination of manpower generation and an event ending. There's at least one exploration event that just gives a huge food boost to your populations, though I forget its name.

If you'd just finished colonizing systems/building warships (particularly warships with invasion modules), that would cause your empire manpower stock to drop, and the food conversion to kick in. I don't think the conversion applies when you're capped out.