Onigiri can fall apart for a few reasons. First, using the wrong type of rice can cause it to fall apart because it's not sticky enough. Use short or medium-grain rice and avoid long-grain rice. The second reason can be that it was not firmly pressed into its shape. Be sure to use firm pressure so there are no gaps in the rice. Third, using high-moisture fillings can cause the rice to separate and fall apart. Be sure to use low-moisture fillings and don't overfill.