In the war games she was involved in, the subs fire a yellow flare to indicate "BANG YOU'RE DEAD" to the carrier. This means that the sub has successfully infiltrated the battle group and obtained the final fire control solution needed to take the shot.
So maybe... just maybe... the BG commander was p**sed at having been sunk and decided to cause some embarrassment to the sub
skipper?
Those airedales just HATE it when they get sunk....