Looks like someone is doing some macflooding (see http://www.securesphere.net/download/papers/SwitchSniff.htm ):
MAC Flooding
Switches keep a translation table that maps various MAC addresses to the physical ports on the switch. As a result of this, a switch can intelligently route packets from one host to another, but it has a limited memory for this work. MAC flooding makes use of this limitation to bombard the switch with fake MAC addresses until the switch can't keep up. The switch then enters into what is known as a `failopen mode', wherein it starts acting as a hub by broadcasting packets to all the machines on the network. Once that happens sniffing can be performed easily. MAC flooding can be performed by using macof, a utility which comes with dsniff suite.
[root@tachyon dhar]# macof
77:6b:e1:6e:5e:8c 93:2d:ed:45:f9:e3 0.0.0.0.45702 > 0.0.0.0.11000: S 1847390231:1847390231(0) win 512
84:a4:d3:57:ef:8 12:56:52:42:dc:95 0.0.0.0.16630 > 0.0.0.0.3031: S 1484147693:1484147693(0) win 512
88:f0:9:3f:18:89 d:86:53:53:d7:f8 0.0.0.0.15535 > 0.0.0.0.7466: S 293820390:293820390(0) win 512