Similar possibility without actual hacking of C&C channel encryption is that the drone (or its windows command machines in Nevada) is just plain infected with a drone virus (like it was recently reported) which has a code "if (it is a steath drone flying over Iran) then (land)"
I can imagine a number of potential scenarios for bringing down an aircraft like this, ranging from full control takeover to interruption of communication until fuel exhaustion and partial crash.
Or perhaps we've bugged it and let them have it. Think we might want to know where they'd take something like this?
so, operator behind windows machine can issue a command to the drone using keyboard/joystick/mouse attached to that machine, yet virus residing in the same machine can't? How come? Does the drone recognize received commands as issued by virus vs. issued by operator ? By smell or a smily at the end or the word "Please" at the start of the command :)