This problem is really easy to reproduce:
1) Enable the SNMP Poller on service-configuration.xml and restart OpenNMS.
2) Add some nodes to a requisition
3) Add a policy to enable the SNMP Poller on all the SNMP interfaces (for facilitate the tests).
4) Be sure that the target node meet the conditions for sending the event for at least one interface (operStatus=down + adminStatus=up)
5) Sync the requisition
After synch the requisition, you should see an interfaceOperDown event for every interface that match the condition (operStatus=down + adminStatus=up).
Every time you sync the requisition or send a forceRescan on the node in question, you are going to see the interfaceOperDown event en each interface that match the condition.
In certain occasions, the event is sent twice (which is even more weird).