Throttling
If throttling usually happens in your backup window you are using more concurrency than the one you are allowed to in your EC2 environment.
Possible solutions are to:
Reduce concurrency with the different strategies mentioned in Best Practices section
Increase throttling limits: https://docs.aws.amazon.com/AWSEC2/latest/APIReference/throttling.html#throttling-increase
Go back to: Troubleshooting.