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I think the tug is being "towed" but has not lost power. I would bet the that tug is in and out of reverse slightly giving the ship more straight line stability in tight quarters at lower speeds where they have lower velocities over the rudder making it less effective. Rudder force is proportional to local Velocity**2
By the tug pulling from the stern the ship can increase rpm a little without going too fast and allow for better tracking and control at low speeds and help control the stern if it starts to go off track by backing down and pulling to the side slightly.
It is possible that the tug is being towed without power but unlikely. That would be my guess. The escorts are probably a safety measure because of the cargo and proximity to congestion.
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