I used to smoke corned beef at a restaraunt I worked at. All I did was sprinkle some pickleing spice over it and smoke it @ 225F for 3 hours then I would wrap it in plastic wrap and cook it for 3 more hours. It always came out fall apart tender. Thinking about your question now. Perhaps you brought the temp up too quick, or it could be the brand you bought like Thirdeye said.
BBQ is the rhythmical creation of beauty in smoke. Its sole arbiter is taste.