Yeah I tried in different USB ports and it still happened. For now I have removed the least important USB devices to see what happens and so far its going okay. So perhaps my first suspect was right that its the last USB ethernet adapter I added. It does run quite hot and the first few times I plugged it in, it didn't work, so its not beyond reason to suspect its defective.
That is one big drag of using USB sticks, if any USB device starts playing up for whatever reason it seems to cause the whole subsystem to go awol. But I really don't want to use a HDD due to the noise and heat they give off plus as I said before it makes upgrading somewhat more complicated.
Ideally I would use a small 4GB SATA SSD with two copies of OpenWRT on there, if I could easily switch between them. Thing is small SSDs aren't particularly cheap unless you get them from China where you can't be sure you got a real or counterfeit one.
I assume there is some way I can manually hack together a working system using the individual kernel and filesystem images and use GRUB to boot between two different installations?