DIAGNOSIS: Something is jamming the blade or causing it to spin off-center. This is usually a hard, frozen item (like a whole ice cube or a large chunk of frozen fruit) that got wedged under a blade.
FIX: Stop it immediately. Unplug the base. Take the cup off and inspect the blades. Remove any obvious large, solid chunk. For a proper blend with frozen items, you must use the extractor blade (the one with the four flat, dull wings) and always add liquid first. The flat wings pull contents down into the sharp blades. Never use just the milling blade for smoothies.
IF THAT DIDN'T WORK: The blade assembly itself might be damaged from a previous jam. Spin the blade by hand (carefully!). If it wobbles or grinds, the bearing inside the black housing is shot. You'll need a replacement blade assembly.
HEADS UP: That violent shaking is terrible for the motor mount. A few episodes like this can permanently loosen internal components, leading to a persistent buzz even during normal use.