Lead/Case assignment rules reassigns the assigned leads/cases from the individual owner to the Queue.

Lead/Case assignment rules reassigns the assigned leads/cases from the individual owner to the Queue.

Assignment rules in Salesforce are very handy when it comes to managing the cases and leads in the incoming queue. However,sometimes it can be tricky to manage the assignments as the assignment rule will be triggered every time the record is edited and the record's check box  “Assign using active assignment rules” is set to true. What happens is that the record already accepted and owned by the specific users is put through the assignment rules and is reassigned to respective queues based on the criteria. 
The workaround is to instruct users to uncheck the box on every edit. Apparently, this is not a user-friendly solution as the users normally forget to address ending up the record to flow through the assignment rules again.

The easiest way to overcome this is to set the “Assign using active assignment rules” check box by default and hide it from the page layout properties. By doing this the user experience of managing the checkbox would be appreciated.

Even with this it does not solved the issue with assignment rules. Though the checkbox is invisible, it is set and thus the record flows through the assignment rules behind the scenes.

The solution is to modify the assignment rules to bypass the record edits. Add the following step in the assignment rules with 1^st order so that any record edit would be skipped from reassigning.

The first order assignment criteria must be 'NOT(ISNEW())' and the Assigned to must be 'Same user'.