This could be because your mixture is too wet or too textured. You need to make sure your potatoes are completely dry and ideally cold before mixing with the other ingredients. Also, make sure to fully drain the tuna so you are not adding extra liquid to the mixture. If you have large chunks of tuna make sure to break them down when mixing. In addition, make sure to chop the onion small. It's important that the oil is hot enough when the patties enter the pan. If it's too low, they will soak up the oil and become soggy. If still falling apart them try baking over frying.