Nico,
It would be very interesting if this happens again to put a sniffer on the net (maybe use wireshark or something equivalent) and see what kind of traffic is going to the switch management address. If the only thing talking to the switch is NetSight, then you might check to see if NetSight is doing some kind of configuration dump of the switch.
Is your switch management on its own VLAN or is it mixed in with user traffic?
James