Comment 11 for bug 1435784

Yep, we don't support dualboot on Nexus 7 anymore, it has too small partition for recovery where Ubuntu boot.img does not fit anymore.
So when you try to reboot to Ubuntu it will fail. Only way out would be some sort of repacking of Ubuntu boot image on the device and slimming it down to fit in Nexus 7 (2013) recovery partition.
I think there is probably existing bug for this.