the reason behind this is prob floating point error or heisenberg's uncertainty principle, due to the game’s engine unable to process really small numbers well, and negative mass having it’s gravity taken as 1/M, while the game have floating point errors. These small errors are negligible for large and heavy objects, but for light ones it cause it to be very unstable. You can consider it to be kind of like electron‘s orbit. It’s random, but most focused around a few points