Simply put, it pulled apart like rubber because it didn't have enough time at the proper temperature to break down the connective tissues. If you're smoking at 225-250F, that cut of meat needed to get to a bare minimum of 190F before being foiled, but 200F would have given it even more love.
Now, to get a bit more complicated...
When you smoke meat, you're braising it. No, you're not surrounding it by liquid, tossing it in the oven with some vegetables, and making a stew...but you're still braising, by even the most technical definition.
When muscle tissue like pork shoulder reaches 130F, the myocin molecules that make up the long stringy strands of the meat start to wind up, like a rubber band on a toy airplane. Since they're full of water, they start to release it into the rest of the tissue structure as they wring themselves out like a sponge. This is a slow process that is just barely starting at 130F, and the hotter you take the meat, the faster that water comes out.
When that water is released, it starts braising the meat from the inside. Water transfers heat more quickly than air, so this water begins to act more quickly on the collagen molecules, which are responsible partially for holding the muscle tissue together, and primarily for connecting it to tendons and cartilage. At 150F, the collagen molecules begin to slowly change from a fiber into gelatin. That gelatin absorbs fat and water, and accounts for meat that is pullable, or "fall off the bone" tender.
Taking the pork shoulder slowly to 200F means it stays in the "Zone" for the appropriate amount of time for all of these changes to occur. Collagen conversion happens between 150F and 200F, with faster conversion happening at higher temperatures. Above 200F, the ribose in the meat begins to brown, starting the Maillard reaction. You want this on the outside, but not on the inside. Maillard reaction + slight pyrolization = bark.
Resist the urge to remove the shoulder from the smoker until its internal temperature reaches 200F, and you will be rewarded with delicious, easy-to-pull pork.