If you thermometer probe is set at the right depth, when I use to monitor the temperature rises more closely, and later I got a Stoker which enabled me to chart the temperature while cooking, I discovered there are usually two stalls. One occurs around 140°F, which I believe is when proteins denature (change structure) and release a lot of moisture quickly. This is often observable when cooking poultry, and this stall is very short - less than an hour.
All meat muscle has connective tissue, but tough cuts like brisket and pork shoulder have a lot of tough connective tissue. This is the tissue that Johnny is referring to. The stall occurs around 160° (some sources say as low as 150°), and this stall can last for a few hours. If you want to "power" through the stall, when the meat hits around 160°F, wrap it in foil. This will shorten the stall considerably.