The plateauing is caused by a phase change; the collagen and fat in the meat are changing phase from a solid to a liquid. One must supply energy to accomplish this phase change. While the fat and collagen are absorbing the energy (heat) from the smoker to melt, none is available to raise the temperature of the meat.
It is similar to ice melting. When ice and water are in a glass, the water will remain at 32F (0C) until all the ice is melted, then the temperature will increase. The heat goes into melting the ice, not raising the temperature of the water.
Nothing mysterious, just thermodynamics. - Unofficial Forum Physicist