Without more information, I can only guess a misconfigured /etc/hostname.<interface> file.
Here is the applicable code from the 4.3-release version of /etc/netstart:
Code:
"dhcp")
[ "$name" = "NONE" ] && name=
[ "$mask" = "NONE" ] && mask=
[ "$bcaddr" = "NONE" ] && bcaddr=
cmd="ifconfig $if $name $mask $bcaddr $ext1 $ext2 down"
cmd="$cmd;dhclient $if"