Question:

Why is an Event record locked after closing?

Answer:

By design, the Event records get locked when the status is Closed_vod:

• The event and all child records (excluding child events) are locked by a trigger.
• The event status is changed to Closed_vod and the Lock_vod field is set to true.
• Any other button with an Event Action that changes the status to Closed_vod will also behave in the same way.
• The event cannot be edited through the UI or through the API. 

Closed Events can be unlocked with the Unlock_vod button:

• The unlock button is displayed if it is defined in the Visible_Buttons_vod field on the Event Layout record, the current Event status is Closed_vod, the user has edit privilege to the Lock_vod field, and there is an event action defined for how the status should change after clicking Unlock.
• Selecting this button unchecks the Lock_vod field and changes the status as defined by the Event Action.

Related Documentation:

CRM Help Documentation: Defining Event Flows > Special Behaviors