Because university policy is such that this can happen.
One example of this often occurs as appointees become eligible for the 8 Year Limit action. When this happens, the Eligibility page in Opus will often show these appointees as being eligible for other actions, the Promotion action among them.
In this scenario, the 8 Year Limit action and the Promotion action should each be started with its own case. These cases may be started at the same time. One way to avoid doing them at the same time would be to do the Promotion action first.
See Also
Can I Start Multiple Types of Cases/Actions Simultaneously?
Can Multiple Actions Use the Same Case (Dossier) in Interfolio?