Meatballs need good binding agents like bread crumbs and eggs mixed with the meat to keep them together during the cooking process. Browning the outside of the meatballs in a frying pan with adequate hot oil helps to seal in the juices and keeps them from falling apart.
Dicing or mincing all added ingredients, like onions or mushrooms, to the smallest size also helps meatballs stay together. Completely combining eggs and bread crumbs or tapioca into the meat and then pressing firmly into meatballs helps them hold their shape. Leaving room to turn the meatballs in the hot oil cooks all sides evenly and holds everything together.