This is what likely happened; Some type of localized EMP or something else like a "Hack" temporarily knocked out the ships electrical systems, who ever was responsible for the ships controls going down also remotely took control or the tankers controls and remotely controlled the ship and used it to ram the USS Fitzgerald.

It was done to send a message, maybe to prevent war, or in attempt to start one. Regardless our government would never tell anyone if they were compromised like that, because it would show how venerable its military has become.

US Navy ships don't have accidents in the open ocean, Tankers don't accidently crash into navy ships in open oceans either. All the pre set navigation paths are known, if something went wrong with a ships pre determined path any navy ship in the area would know immediately and be able to take counter measures. Its considered an attack to use a tanker as a battering ram and the Navy would perceive it as that.

Thanks Evo, for backing me up, and doing a little research, but you probably already knew this before I posted it.