Plaque? No one really knows. The most common thought is that the area of the artery afflicted with even minor plaque is more prone to spasm because it loses its ability to dilate properly. This is only a hypothesis really.
Paradoxic. Constriction of blood vessel. This can be related to adrenaline or lack of endothelia derived relaxing factor which is now nitric oxide. Normal arterial lining can release no but diseased arteries can not do this.
Mostly unknown. Coronary vasospasm is a rare condition. It can lead to severe chest pain mimicking a heart attack. Generally the spasm relents quickly but heart attacks and dangerous arrhythmias may occur. Caffeine may be a trigger and should be avoided. Illicit stimulants such as Cocaine would be an even more potent cause. Spasm most often occurs with no identifiable cause. Preventive medications exist.
Stress. Usually extreme stress but these are relatively rare & can often be treated with medicines.
Adrenaline. Stress, and sometimes spontaneously. It is inability of the vessel to release Nitric Oxide and thus dilate with paradoxic constriction.