In a straight-up WC case, the actual cause of the accident may not be significant to the case's final resolution (that's what the insurance is for). In the WC/Medicare cases, however, causal factors may matter a lot.
Under the Medicare Secondary Payor Act (MSP), Medicare funds are deemed 'secondary' (to be used after primary funds are exhausted) to those of other, non-employer entities if they contributed to the cause of the injury. However, that 'secondary' classification can't be made until those contributing factors are established, and that determination may not be made for days or even months after the incident. The MSP Act asserts that when another entity is deemed to have caused the injury, then it is improper to use Medicare funds to cover the cost of the recovery. Further, when that determination is made, but Medicare funds are already dispensed, then the CMS has a legal duty to go after that other entity to retrieve those funds back.