"A keep-alive is a small piece of data transmitted between a client and a server to ensure that the connection is still open or to keep the connection open. Many protocols implement this as a way of cleaning up dead connections to the server. If a client does not respond, the connection is closed.
SSH does not enable this by default. There are pros and cons to this. A major pro is that under a lot of conditions if you disconnect from the Internet, your connection will be usable when you reconnect. For those who drop out of WiFi a lot, this is a major plus when you discover you don't need to login again."
Source: http://www.symkat.com/ssh-tips-and-tricks-you-need
There's probably better sources out there, that was just one of the top results in Google as, if I'm honest, I'm not an expert on this either.
This happens because your router or firewall is trying to clean up dead connections. It's seeing that no data has been transmitted in N seconds and falsely assumes that the connection is no longer in use.
To rectify this you can add a Keep-Alive. This will ensure that your connection stays open to the server and the firewall doesn't close it.
I'm open to being proved wrong here, but as I've already said, only been doing this for several years now, so I'd need a counter argument to explain the mechanics of what's allowing the connection to reattach rather than "it's not possible" :)
edit: hmmm, re-reading the latter part of keep alive article I posted, it does seem to imply what your saying. So how come my SSH connections aren't nuked then? Is this just a property of TCP/IP (I'm not a networking guy so ignorant to some of the lower level stuff)