If your rig has bad internet connection, this can happen. I have very strict timing rules to prevent spending too much time waiting for single mining rig. If connection is not established in 2 seconds, rig is reported as OFF, if data is not sent in 2 seconds, rig is reported as OFF.
Hmm, I see... My internet connection is definitely OK, I got cable and it's usually very stable. Also CGminer didn't report anything(bad connection, etc...) when I saw the "OFF" on the website. These 2 seconds will not pose a problem when your service becomes more popular? What about DDOS attacks?
If there is DDOS attack on website, then all rigs will be OFF.
Keep in mind that it isn't just quality of your connection. If you do some heavy duty downloading or uploading, that can higher your latencies, thus can show your rig as being OFF.