Hi Saquid
As I said, I am really not aware of any standard setting for this. In fact, I am seeing this for the first time that the system is explicitly asking for a reserve of the workitem!!
All I gave you are hints as to where a custom enhancement may have been done to give this popup box.
If it is happening for all workflow tasks (of different task IDs), chances are that there is some enhancement done in reserve functionality. If it is happening for all work items of a single task ID (belonging to a single workflow) , chances are that there is a program exit plugged in the step.
Regards,
Modak