I don’t understand why that’s the case - I thought flows of goods were really consistent in normal years?

Depends entirely on when the ships show up!

ships bring lots of goods at once. if you get something near PANAMAX size delivering a shipment of cars, you'd better have people who can do RORO until the ship is empty. So those folks get lots of work all at once. But that kind of ship woth that cargo doesn't come along every day. For instance.