the last breakdown i saw about ships waiting to get in was basically "there's nowhere to unload" since demurrage space is full. lack of truck capacity is keeping stuff stuck in demure. train capacity is full, and no truck containers to get from port to train.

it's essentially the "spring traffic jam" scenario where with enough density/volume, every movement just stacks up again, then creeps, then jams. (think highway traffic with no exits - tap of the brakes up front causes the rear to come to a complete stop and it compounds)

https://youtu.be/cEdKaEVNg3M