This is caused by the golgi tendon reflex. This small group of nerves resides near the end of the muscles where they attatch to bone. The musles cause the shaking to stop the possibility of causing damadge by continuing the motion. the shaking will get so bad that you have to stop what you are doing. this will continue until you have built neurological pathways that tell the reflex action that this weight is ok. that may take a few workouts or several depanding on the person. I still experience this, even after 20 years of training, if I am attempting a new weight. As my body is used to being asked to constantly adapt to new stress I usualy find that one or two workouts later I fell stable again.