I am not sure if this can be treated as an issue as this history message indicates that the current assignment is removed by ticket. This occurs when the ticket is raised (either through an activity or SLA or any other means) and it jumps to another shape resulting in deletion of existing assignment. You need to look at the configuration of your application to check what all are the places where tickets are getting raised.
SLA rules would be probably a good place to start.
rawap, I see there is one SLA for that assignment, however i don't see any code to write history like AssignmentInturrupted in it? How this automatic trigger of SLA is happened based on some property like pySLAGoalTime of the work object? Please let me know the steps to find out the root cause of this issue.