Container ports periodically accumulate stacks of empty boxes while shippers elsewhere cannot find one. The imbalance is structural, and it follows directly from the fact that trade does not flow equally in both directions.
Trade lanes are not balanced
A route carrying manufactured goods one way and raw materials or agricultural produce the other does not require the same number of containers in each direction.
Bulk commodities frequently travel in vessels rather than boxes, so the container arriving full has nothing comparable to carry back.
Boxes therefore accumulate at import-heavy destinations and become scarce at export-heavy origins, without anything having gone wrong operationally.
Repositioning costs money nobody pays for
Moving an empty container back to where it is needed consumes vessel space, handling and inland transport, and generates no freight revenue at all.
Carriers recover this through the rates charged on the profitable leg, which is why the same route often prices very differently in each direction.
The imbalance is therefore visible in freight rates as much as in the stacks at the terminal.
Storage decisions depend on relative costs
A carrier chooses between storing empties near a port, moving them inland to a depot, or shipping them back, and the cheapest option changes with demand.
Where terminal space is scarce, storage charges rise sharply and repositioning becomes worthwhile even at low expected utilisation.
Where space is cheap, boxes sit until demand returns, which is why accumulations can persist for months without resolution.
Disruption amplifies the effect
Delays anywhere in a network hold containers in place longer, so the same fleet performs fewer round trips in a year.
Effective capacity falls even though the physical number of boxes has not changed, and shortages appear at origins that were previously adequately supplied.
Recovery is slow because the fleet must be redistributed while continuing to serve ongoing demand, and the two objectives compete for the same vessel space.
Leasing absorbs part of the mismatch
A large share of the global container fleet is owned by leasing companies rather than by the carriers using it.
Leasing allows a carrier to take boxes where they are plentiful and return them where they are scarce, which converts a physical problem into a contractual one.
Drop-off charges and location restrictions in those contracts are how the underlying repositioning cost reappears, since the imbalance itself does not disappear.