The basic reason for "reperfusion injury" to the myocardium is:

After MI, there is failure of Na+-K+ pump resulting in NCX failure Thus intracellular Ca+2 increases Now when perfusion is established (nutrients are available), because of increased ICF Ca+2, cardiac fibers contract very strongly Already muscle fibers are fragile due to M.I, now this contraction results in fuher damage. This is basic reason for "reperfusion injury".