Possibly due to MAC flooding attack - a kind of Unicast flood attack where an attacker would exhaust space in the MAC allocation table of a switch, so not being able to find the right address in the address cache (all were supplied by the attacker), it would flood the packet out to all ports (if a switch received a unicast packet with a destination address unknown to it, the packet is treated like a broadcast packet and is sent out to all hosts on the network). After most/all/target legitimate MACs were forced out of the table, it's a nice time to start a packet sniffer as the previously unavailable packets that were spat out by the switch are purportedly available for capture. Even if no packet capture intended this is already a type of DoS attack, esp. in large networks (due to the number of clients talking to the switch), which certainly degrades network performance.
BTW, unicast ARP requests are legitimate as per RFC1122 which states on Page 23:
(2) Unicast Poll -- Actively poll the remote host by periodically sending a point-to-point ARP Request to it, and delete the entry if no ARP Reply is received from N successive polls. Again, the timeout should be on the order of a minute, and typically N is 2.