The simplest reason your meatloaf falls apart is that you likely tried cutting it when it was too hot. Meatloaf is easier to slice after you've let it cool for at least 10 minutes. Room temperature or refrigerated meatloaf is even easier to slice. Another possibility is that you didn't use enough binder, like eggs or cracker crumbs. Next time you make it, try increasing your cracker crumbs by a small amount.