A lot of websites use a cache in front of databases (or template rendering engines, or many other systems). That cache might evict entries based on time - after 5 minutes, the entry is considered invalid.

But that means that if you have no traffic for 10 minutes, the cache completely empties. Then when traffic returns, it all skips the cache and actually triggers a real hit to the backend - which is now overwhelmed with traffic. The cache protects the backend in normal behavior, but now it's not doing its job, so the backend has many more requests than usual.

In the worst case, those requests are enqueued in a big serial sequence... but the ones at the back of the queue may time out. The client may do something like say "it's taken me 5 seconds and I still don't have a response - I'll abort and retry!" and now you have even _more_ traffic to deal with.

So cold caches and retries can conspire to keep a service down for a long time even after the root cause is fixed.