Saved-for-later then reheated potato mixture sparks and catches fire in microwave

I have a bird who gets medicine twice a day. I hide it in a mashed potato fruit mixture. I make enough that the potato mixture lasts several days and add the medicine as I need it. I take a small amount and microwave it for about 8 seconds (just enough to soften it). Then I normally add a couple blueberries to the mixture for a little more moisture and flavor. At that point, if I feel like I've made too much for that "serving" I'll put some back in the container for next time. When I reheat the saved-for-later mixture, it always sparks and catches fire (about the size of a lighter flame) in the microwave (it always goes out as soon as I stop the microwave). Of course, I never use the caught fire serving and always start over. This only happens with the saved-for-later bits and not the original mixture. I read the "Why did my banana catch fire in the microwave?" (Why did my banana catch fire in the microwave?) thread and that might answer it if it were all of the mixtures that caught fire, but it's only the parts that I tried not to waste and save for the next medicine-hiding dose. Any ideas what causes just the left-over bits to catch fire?