Hey,
Following are the scenarios may be create issue -
1. Make sure Dn has been gone in notready mode , there are possibilties while you try to set notready mode its sometimes take times to sent the query to sip server and stat server meanwhile if the call flow level high then there is chances to get call received on these mode.
2. Check network performance and system performance as well .
3. Check in sip log what is the state of dn at that particaular time. make sure check logs before 2 min and after 2 min it will help you to understand the dn status at that time.