Preseed doesn't automatically select network interface on Ubuntu 14.04; automated install needs human input

This is (or was?) a reported bug. The solution was to try passing netcfg/choose_interface= YourInterface to the kernel as a boot option.

According to this article, you can edit pxelinux.cfg in append line for multiple interface issue when using preseed.

by interface name

append initrd=/images/Ubuntu/initrd.gz ksdevice=bootif lang= interface=eth1

or with mac address

append initrd=/images/Ubuntu/initrd.gz ksdevice=bootif lang= interface=90:e2:ba:2e:b0:70