Default [Q] Nexus 5 - Disabling USB Tethering hinders debugging

I have a very strange issue with my Nexus 5 (Android 4.4.2).

I have USB debugging enabled since day one because I'm using it as a development device. Until today it worked all fine. Today, I enabled USB Tethering using the menu Settings > More > Tethering > USB Tethering because I needed Internet connection. After a while I disabled it. From that point on, my ADT Bundle didn't see my device anymore! Strangely enough, as soon as I enable USB Tethering, my PC sees my Nexus again! Can anybody explain this? It looks like a nasty bug...