I think the whole ship went dark for a moment. That may have had an impact on the rudder, and then some strong current and/or wind, pushing it sideways.
Was watching CNN around 6AM EDT, they had a reporter on scene who mentioned wind "whipping across" the harbor. This was in reference to the potential survivability of the freezing cold water, but it seems likely it could have been a factor in pushing the ship off course as well.