Part of it is additional load. Part of it is their move of more and more of Github infrastructure to Azure.

I've done a lot of "plain compute" work [0] with the Big Three Cloud Compute vendors. Azure is by far the worst. Mysterious resource creation failures, mysterious resource deletion failures, mysterious "incompatible schema" failures when talking to Azure provisioning and status infrastructure, mysterious and inexplicable performance problems, etc, etc, etc. Unless I was being paid a lot of money to use Azure, I'd take Google's legendarily nonexistent support over Azure's unreliability any day.

[0] That is, "create a VM with persistent disks, Internet access, and maybe a load balancer in front and ignore all of the other features provided by the vendor"