What will cause "Expired" in the status of K2 work

When go to K2 workspace, in the status column (see attached jpg file), sometimes users will see "expired".

As we understand, if the task has been worked on by other people in the destination group, the status will show as "expired".

But sometimes, there is only one approver in the destination group, the task has not been worked on by the user, who is the single approver in the destination group. It happens that the user will see the status as "Expired". He/She reports this to us. After we reroute the task to the user again, then the status will show as "Active".

So we don't understand under what circumstances will this second scenarion happen. We are trying to reproduce the problem before we can solve it...

Thanks
coenw