Some switches seem to have a multicast problem: we've downed very fancy cisco switches recently (can't recall the number right now) with igmp/multicast traffic.
We've had a couple of hundred embedded systems that were announcing themselves on the network with mdns. That in itself is only a very low amount of traffic.
Probably some management software triggered a slight increase in reporting, but with a couple hundred embedded systems, this was enough.
mc traffic/igmp does not seem to be hardware accelerated; being routed to the main switch CPU -> maxed out.
Disabling mdns 'solves' the problem.