The container ship was being driven by computer? USS Fitzgerald was struck on its starboard (right-hand) side, and maritime rules require vessels to give way to others on that side.
Personnel on destroyer asleep at the wheel?
I was Army so I know nothing about Navy vessels. However, I assume there are redundant alarm systems if ships get too close? Should the bells and whistles all have been blaring before the crash on the Fitzgerald and the tanker???