The only circumstances I could think of in which a ship of this size would require an escort tug would be some mechanical malfunction or deficiency. This could range from a missing anchor, defects in steering, radar or bow thruster or some other component. Some of these might be serious for a ship in windy conditions and crowded waterway. So the call for an escort tug would be a sensible precautionary measure, but would also be required by regulations.
A second tug has joined, and the escort tug has moved to the port quarter to turn the ship off Halterm on arrival this morning.
Oceanex Sanderling (and its fleet mates) operate under a demanding schedule. Today's is the ship's second call in Halifax this week, running almost non-stop to and from Newfoundland. Wear and tear must be an issue eventually, and minor issues are bound to arise.