At this point in the journey, they were cleared of the docks but not the channel. There wouldn't be a docking pilot on board nor would there be any tugs. It was cleared and under it's own propulsion until it wasn't.

My sailors guess from the footage and the reports is diesel generator failure(s) resulting in loss of power, restoration of power, then loss of power again. Bypassing the diesel generator (which provides power to hydraulics too) and manually throwing the engines in reverse. The billow of black smoke. This could have possibly burned out an engine, blowing the camshaft or propshaft or transmission.

The reverse was too late as the ship was already heading for the bridge pylon. Even at full reverse, you couldn't slow it down fast enough. Tragic.