A before and after shot might have been informative.

I'm guessing that the meatballs were touching before cooking? Minced meat typically contains quite a lot of fat, so meatballs shrink a fair bit on cooking. This means that they occupied the only space they could when uncooked, and shrank away from one another during cooking.

Just a guess.

I agree with Pete, including the bit about guessing, and assuming no hanky panky and not experimenting myself.

More exotic possibilities could be reasonable though (still handwaving and guessing of course!)  There could be an influence involving the interference patterns of the microwaves (though I admit it doesn't look very much like such a pattern. It is of a suitable scale of size though.)

Apart from the fact that they might contain fat, they also might simply have contracted their proteins and ejected juice. This should tend to push them apart, and away from the sides of the pan, though whether strongly enough, I could not say.

It could be just that their presence in the juice caused local heating that moved them away from each other.

It also could be that even heating of the gravy caused the development of Bénard cells, but that is a long shot, I think.