One that does not expect the unexpected are unprepared.

It's to do with your body's reaction to fear, you get a rush of the hormone adrenaline which makes your body more able to react quickly in a stressful situation. Adrenaline can speed up your heartbeat, dilate the pupils so the body is more able to "fight or flight". One of the problems with this biological response is that it can occur when you dont really need it to, for instance if you are frightened but can't do anything about it, e.g. seeing a spider on the tv, after an accident or you are suffering post traumatic stress disorder when you remember a traumatic event. shaking can also be a sign of shock after a traumatic event.

This is a built in defensive mechanism that makes people harder to shoot when they are shaking and thus moving around so much. lol

Shaking is the way the body works out the deep muscular tension caused by a traumatic reaction. See this interesting website for more info: www.traumaprevention.com

Animal's have been known to fake a convulsion indicating to a predator that their consumption might not be the healthiest of decisions. As a creationist at heart I see no human parallel, but I thought it worth mentioning.

During such a situation, your body, which is controlled by two major nervous control systems, switches from parasympatheic control, to sympathetic control. More accurately, the emphasis shifts from one to the other, since both are working continuously, just that one presides over the other. This has many effects, one of which is the release of andrenaline form the adrenal glands (just above your kidneys). This hormone acts on many other systems as described by others, notably the eyes, making them dilate to increase the amount of light entering, so you can see the danger clearly, and increasing your heart rate to allow supply of oxygen to your muscles to allow them to work hard enough to escape quickly if necessary. Related to this, the body switches from storing glucose (a basic sugar) as other chemical energy stores in your tissues, to releasing it from these stores to provide the muscles with the energy to work hard in conjunction with the increased oxygen. Now your muscles have the increased oxygen, and energy available, and they are well stimulated by your nerves, so that you are 'on edge', the fibres twitching, warming them up so allowing you to start running quickly and make your bolt for freedom. However, you also lose fine control over your muscles to a degree as the area of the brain that deals with fine motor control is different to that which deals with the raw, basic survival needs, since it developed later in evolution. So you can make gross (big) movements, e.g. your legs running, but you have a reduced ability to type on a keyboard because you have lost some finer control, which normally rules over your muscles to help reduce shaking. All in all, the stimulus and chemicals in your body have suddenly shifted drastically, and just like if you drive a car normally, then push it to it's absolute maximum on a motorway, the movements become more erratic and harder to control.