The deployment plan is configured to use automatic rollback but the rollback deployment is awaiting user/manual intervention to start the rollback job - in either "Initialization Pending" or "Validation Pending" status.
The reason why it is showing "Initialization Pending" or "Validation Pending" status is because the "Environment" has not been explicitly set to allow Automatic rollbacks.
The reason why it may show "Initialization Pending" sometimes or "Validation Pending" other times is based on whether or not a Pre-Deploy step has been added to the deployment plan that the rollback is based off of. If a Pre-Deploy step is present then it will show "Initialization Pending" until the Environment is configured to allow Automatic Rollbacks. Either that or the rollback settings in the deployment plan are set to "Manual" invocation.
CA Release Automation 6.5 and above.
Go to the Environments -> Rollback view. From there select the application and specific environment you want the automatic rollback to start for – without any kind of user/manual intervention. Then click the edit link to the right of “Environment Level Settings” and select the radio button for “Allow manual and automatic invocation”
Full overview of getting the rollbacks to work automatically: