Can anger or exercise ever precipitate a heart attack?

Yes. Heart attacks (myocardial infarctions) are almost always believed to occur when an atheromatous plaque ruptures. The exposure of this material to the blood stream triggers the coagulation cascade, and the resulting thrombus occludes one of the coronary vessels. The tissue that depended on this vessel for perfusion will therefore soon die (if the thrombus is not removed or degraded), and the result is a heart attack.

Exactly what causes the atheromatous plaque to rupture is a matter of intense research at the moment - after all, it may have been sitting there fairly benignly for a decade or two. One of the logical candidates would be the flow rate through the coronary vessels. The faster blood rushes around the bends of these vessels, the more likely the blood is to tear off a bit of the plaque (technically, this would be due to increased "shear stress" on the atheromas).

During strenuous exercise or heightened emotional states (like anger and, sadly, sex) the heart is asked (by the sympathetic nervous system) to pump faster and more strongly. In order to facilitate this, the coronary vessels dilate to allow more blood through per second - with the intention that this blood will provide extra energy for the heart's increased demands. Unfortunately, very rarely this has the undesired effect of rupturing an atheromatous plaque that a patient might have, and thus potentially precipitating a heart attack.

One final thing: this does not mean that exercise is bad for you! Even if you do have coronary artery plaques, mild-moderate exercise will reduce your risk of having a heart attack. It lowers your blood pressure, it lowers your weight and it even directly lowers your risk of a myocardial infarction. Just take it easy and don't strain your body too much - intense exercise is the worry here.