If it was failing slowly, why didn't they fix it then? Why didn't they notice?
Could it be because they fired all the people who know how to monitor and identify such problems? That's what SREs are for.
Please share the information you have which contradicts the conclusions I have drawn from Github's statement.
(And we know they're liars. They report very few of the actual incidents they have; see for example https://mrshu.github.io/github-statuses/)
These backlogs can cause clients to make more retries, exacerbating the problem. Potentially further cascading through the system.
The art of large system design is to identify and avoid these kinds of chokepoints. And when something happens, propagate the "backpressure" up the stack to avoid queuing.
AWS got a fair share of similar outages, so the newer SDKs now try to not exacerbate these kinds of issues: https://docs.aws.amazon.com/sdkref/latest/guide/feature-retr...
The original AWS EBS outage is probably the canonical example: https://aws.amazon.com/message/65648/