Did they ever give you an explanation as to why it happened?

1. We changed a configuration parameter, which we thought would be a no-op but caused a deploy

2. We ran a fairly large cluster with a large amount of i3.xlarge

3. One AZ in us-west-1 didn't have enough capacity of that SKU . The other AZ did, but this doesn't help if you have a multi-AZ deployment.

4. We switched to EBS-based instances after this

Reach out to your TAM again to talk about CR and RI combinations which can help to mitigate this problem.

Migrating to a different hosting platform than AWS for ES aslo mitigates this problem too, which is more likely in our case.