Complex sentence: The men who found U- 869 believed that it was a victim of its own torpedo, which may have become a "circle-runner" if a defective steering mechanism caused it to change direction in the water and head for origination point.
Split into simpler sentences: The men who found U- 869's loss is unknown believed that it was likely a victim of its own torpedo. "Circle-runner" is the name for a torpedo with a defective steering mechanism causing it to change direction in the water and head for origination point.