You didn't cook it long enough. You cannot cook a pork shoulder or butt for pulling by temperature or time. The only way is by feel. In my experience, 10 hours at 250F for an 8 pound butt is nowhere near long enough. If it is bone in, you want to cook it until the bone wiggles easily. If no bone, then cook until the meat splits easily apart. Then pull, rest for 10 minutes, wrap in foil and/or pan and cover. Let rest for 2 hours. Cooler if you want, or a warmed oven.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"I'm Armenian...perhaps...but then again...maybe not..."