From OpenNMS

There's another name for false outages: outages.  ;)

Is it possible there's a bug in OpenNMS that's causing "false" outages? Of course. Is it likely? Not really.

False outages are rarely false; they're usually a sign of an overworked network, overworked monitoring system, or some other intermittent issue on your network.

Perhaps your routers have a bad SNMP agent which causes a short outage after scanning? Maybe there's something else going on with QoS or throttling that's causing such things. Or perhaps your OpenNMS server has some I/O issues and it's getting overworked?

Don't be so quick to dismiss an outage as false, it is usually a sign of a deeper issue, be it hardware, load, or agent/client/service-related. Do some tcpdumps, look at your traffic, investigate. That's what outage reports are for. =)

See also: 30 second outage