The network connection is USB also (on the motherboard of course)
You will most likely find in the system log a comment about the network device failing.

On mine I wrote a cron job that runs every minute and checks the network and if it's not working, it send cgminer a quit and shuts down the RPi
The main reason for this is that obviously pulling the power out while the RPi is running is a bad idea, and once the network dies it seems it wont restart without a reboot anyway

Sounds like Raspbian have screwed up something there with the update

The network dropout isn't unknown, just it seemed to have become quite rare at least up until recently.
I've had it happen once on Arch (but only once) but a few times on Raspbian (used to sometimes happen when I plugged in an Icarus pl2303)