Facing union drives, citations from state workplace regulators, lawsuits and shareholder pressure to reduce injuries, Amazon signed on to a new safety pledge Thursday. The next step in its five-year, $12 million partnership with the nonprofit National Safety Council, the pledge is a commitment to reduce musculoskeletal risks and injuries by 25% by 2025. Also known as MSDs, musculoskeletal disorders are ergonomic injuries like back, shoulder and wrist pain that are often caused by repetitive movements, awkward postures or heavy lifting. More than 15 companies have signed on to the MSD Pledge, i…