Transaction fees sit near nothing for long periods and then rise sharply within hours. The behaviour follows from a fixed supply of block space meeting demand that varies without limit.
Capacity does not respond to demand
Blocks have a size limit and arrive at a roughly constant average rate, so throughput is fixed regardless of how many people want to transact.
Unlike most services, the network cannot add capacity when busy. There is no elastic supply to absorb a surge.
Anything demanded beyond that capacity waits in the pool of unconfirmed transactions, which is where the queue that produces the fee market forms.
Miners order by fee density
Miners select transactions to maximise revenue from a limited amount of space, so they rank by fee per unit of size rather than by total fee.
A physically small transaction paying a modest fee can therefore outrank a large one paying more, which is why fees are quoted per unit of transaction weight.
Users compete by raising that rate, and the effective clearing level is whatever the last transaction included in a block paid.
Below capacity the auction barely operates
When demand fits comfortably within available space, every reasonable transaction is included and there is no need to outbid anyone.
Fees then sit near the minimum that nodes will relay, and the market is effectively dormant.
This is why fee levels are not a smooth function of activity. They stay flat across a wide range and then move steeply once the threshold is crossed.
Congestion is self-reinforcing while it lasts
A backlog persists after the surge that created it, because the queue only clears at the fixed rate blocks provide.
Users watching confirmations slow raise their fees, which pushes the clearing level higher and leaves lower-paying transactions waiting longer.
Replacement mechanisms let a sender resubmit with a higher fee, and receivers can pay for a stuck parent transaction by spending its output at a high rate, both of which add bidding pressure during exactly these periods.
Transaction size is a cost users control
Because fees are charged by size, the number of inputs a transaction consumes has a direct effect on what it costs.
A wallet holding many small amounts must combine several of them, producing a large transaction and a large fee, which is why consolidating during quiet periods is standard practice for anyone managing many outputs.
Address formats and signature schemes affect size too, so the same payment can cost noticeably different amounts depending on how the wallet constructs it.