During process of Derrickman (DM) changeover, Floorman (FM) relieved DM and pulled & racked back a stand of pipe. DM was standing close to ladder with fall arrester attached in preparation to descend down the ladder. Driller lowered Top Drive System (TDS) which entangled and pulled the fall arrester line along with DM. DM fell (same level) at monkey board platform with left leg trapped between monkey board post & guide beam resulting in a fracture to his left ankle.