Sometimes p-shakes can be brought on by high negative feelings, and it would follow that at least some of the time the shakes are symptoms of stress. Most of the time, though, they just show up during a high proportion of everyday attempts to manipulate the physical world.
Emotional-stress-related shakes can last an evening or most of a day; these are more annoying than directly upsetting, but prevent anything useful being done. Normal background shakes fade in and out at levels usually barely noticeable, though they increase manyfold as soon as they impair anything enough to stress Mutt. It's pretty much a vicious circle.