@Rodney Tamblyn There is likely an update going on. App Service works within update zones. Zone 1 will be taken offline to be updated. Any sites in zone 1 will be moved to zones 2-10. If your site was moved to zone 2, it will be moved again upon zone 2 starting its update and so forth.
Are you experiencing any issues due to the change of instance or are you curious about the cause of the behavior?