The cost of moving a container across an ocean can multiply within months and fall back just as quickly. The volatility comes from a supply side that cannot respond at the speed of demand.
Capacity is ordered years before it arrives
A large container ship is ordered from a yard and delivered several years later. The decision to build is made against conditions that no longer exist when the vessel enters service.
Owners order when rates are high and profits look durable, so new capacity tends to arrive shortly after the boom that justified it has ended.
The result is a cycle in which the fleet grows fastest precisely when the market least needs the extra ships.
Shipping capacity cannot be stockpiled
A voyage that sails half empty is lost permanently, because the space on that sailing cannot be saved and sold later.
That makes carriers willing to accept very low rates rather than sail with empty slots, which drives prices down hard in a slack market.
In the opposite condition, when there is more cargo than space, the same logic runs in reverse and shippers bid against each other for a fixed number of slots.
Demand moves faster than the fleet
Retail ordering responds to consumer spending within weeks, and inventory decisions amplify small changes in end demand into large changes in shipping volume.
When retailers restock together, the surge lands on a fleet whose size was fixed years earlier and cannot be expanded in the interim.
That asymmetry, fast demand against slow supply, is the structural reason the market has no natural resting point.
Small disruptions have outsized effects
Ships spend a defined number of days at sea and in port, and anything that lengthens the round trip removes capacity from the market without removing a single vessel.
A rerouting that adds days to a voyage, or port congestion that holds ships at anchor, can absorb a meaningful slice of global capacity at once.
Because the fleet usually runs close to full in a firm market, a modest loss of effective capacity can move rates sharply.
Why contracts do not smooth it out
Large shippers sign annual contracts precisely to avoid the swings, and carriers use them to secure baseline volume.
Those contracts hold when the market is calm, but in extreme conditions the incentive to break them rises on whichever side is losing, and enforcement is difficult across jurisdictions.
Rates therefore reconverge on the spot market at exactly the moments contracts were meant to protect against.