What triggers sudden rupturing of plaque in coronary artery, causing mi , is it sudden increase in high blood pressure ? is it the physical exertion of that moment ? or is just lack of luck ?

Inflammation. : Plaque rupture is triggered by a high degree of inflammation within an arterial plaque. This inflammation occurs in what we call an unstable plaque, and is mediated by macrophages as well as bad cholesterol within the plaque. Control of risk factors for cardiac disease can reduce the inflammation in these plaques, making the more stable and less prone to rupture.

Inflammation: Agree with Dr. Gianonne. The enzymes released by inflammatory cells can eat through the cap on the plaque and result in rupture.

All of above: This sudden rupture can happen with out any cause also .keeping up with low cholesterol,bp control as well as regular cardio exercise can stabilize these

Uncertain: Probably multiple causes, depending on the individual. If someone figures this out they may get a Nobel prize.