What made the centurion say, "Truly this man was the Son of God!"?

Mark tells what the centurion saw, in the same verse: "When the centurion, who stood by opposite him, saw that he cried out like this and breathed his last, he said, “Truly this man was the Son of God!”" (15:39) (tap breathed). Mark says no more of why. He doesn't say what the centurion understood by his words. Mark names "the centurion" twice more: Pilate, "summoning the centurion, he asked him whether he had been dead long" (15:44), and "When he found out from the centurion, he granted the body to Joseph" (15:45). Mark began his book with "Jesus Christ, the Son of God" (1:1), and the book's introduction sets the centurion's words beside the disciples' question, "Who then is this?" (4:41). These notes add no meaning to the centurion's words beyond what the verse says.