Russian subs were busy. And not selective about targets.
For people saying the subs should have looked into the ships to investigate it worth remembering that Russia had two active Nazi-backed Islamist insurgencies in the immediate proximity from Black Sea. One by Tatars in Crimea and another in Chechnya. You could easily confuse a boat full of refugees for a Turkish boat bringing rag-head jihadists to help the Nazi and Chechens.