A temporary employee and three to four additional temporary and permanent employees were attempting to down-stack leaning pallets of product (canned and bottled beverages) from a stack of three loaded pallets. The stack was approximately 15-20 feet high. The pallets fell, for an unknown reason, and the temporary employee was pinned under product and pallets against the wall. He suffered a broken femur and fibula, broken ribs, a punctured lung, and a possible lacerated spleen. He was hospitalized.