Hysteresis
The pressure can be gone while the system is still carrying what it changed.
A system can keep the effects of an earlier condition, so removing the original pressure may not bring back the old state.
Removing the pressure may not bring things back
It is easy to assume that a system will return to normal when the thing stressing it goes away. Sometimes that stress changes the system in a way that stays.
That is hysteresis. The route into the current state matters, because it left something behind. Turning the original input back down does not automatically undo the result.
Ordinary lag means recovery takes time. Hysteresis means waiting may not be enough: something was stored, learned, depleted or changed, and recovery needs to deal with that directly.
Why the service stays down after traffic returns to normal
A traffic spike causes requests to time out. Clients retry, retries create a queue, and the queue causes more timeouts.
When traffic returns to normal, the stored work and retry behaviour can still keep the service overloaded. Recovery needs a reset, such as draining or discarding the queue, limiting retries, shedding load or breaking the feedback loop. Even after the spike ends, the system is still busy with the work it left behind.
What systems can remember
Organisations. After months of unpredictable punishment, a team stops raising risks. A new leader can say "speak freely," but people have learned that silence is safer. They need repeated evidence that speaking up will not cost them.
Economics. A recession can leave people unemployed long enough for skills, networks and confidence to weaken. Demand can recover while people remain excluded because the shock changed their ability to re-enter.
Biology and health. Prolonged stress can change regulatory systems after the immediate danger has passed. Removing the trigger and restoring the earlier state are separate jobs.
Relationships. An apology can end an argument while the other person still expects the same thing to happen again.
Find what remains
Hysteresis is more specific than slow recovery. Use it when you can name both what the system retained and what could reset it. Without those two things, you may only be describing a system that needs more time.