"Atlantic Salmon" is farmed salmon. Wild Atlantic Salmon, from what I've read, is commercially extinct.
Without getting into a discussion of whether you should be using unfrozen, salmon (either farmed or wild) for raw preparations (which you shouldn't), salmon that is mishandled will tend to fall apart along the seams or grains of its flesh. Also, salmon that has been pre-cut into fillets will tend to fall apart quicker because the meat will stretch and bend when handled, causing the meat to fall apart.
Your best bet for any ceviche, crudo, or sushi type preparation is to buy whole fish, break them down yourself at home. Keep the fish whole will, at a minimum, minimize flesh degradation by (1) minimizing exposure to air, (2) keeping the physical structure intact (i.e. the bones of the fish keep the fish from bending, twisting and falling apart).
Lastly, many fish processors, especially for large fish like salmon, put the fish through a de-boning process to pull out the pin bones. This, IMHO, is the quickest way to cause the meat to fall apart as the pin bones in the fish hold the meat together. Pin bones should be pulled out as close to the time that you intend to prepare the fish as possible. Notice how the flesh looks before and after the pin bones are taken out, even with super fresh fish that's in rigor mortis. Even super firm, fresh fish will suffer tears as soon as you pull the pin bones out.