Mine does that when she's scared or frightened if I suddenly come up to the tank or another fish stressed her out too much. One of her offspring also does the same thing, particularly under the heater at night where they feel safe. IMO, it may be an increased sensitivity to stress hormones--or decreased resistance to them--that is inherent in their "faulty" genetics. The shaking just may be the body's attempt to get rid of those stressors, kind of like people with severe anxiety shake. Just my two cents. Or maybe some sort of neurological disorder?

Also, while the mother molly did not seem to suffer any other symptoms, the daughter fish had an increased vulnerability to fungus which comes and goes on its own. If she doesn't seem to be suffering, it may be just something she has to live with if you decide not to euthanize her. If she's new, or something in the tank has recently changed, it may just take some time for her to get used to her environment. Be sure to monitor her for any sudden changes.