I've had my Sherman drop into "testing" mode by turning the TX on and off while the tank is on. I was horsing around in my living room with a family member but my TX battery was low, so I'd shut off the TX for the 15s it took for the tank to restart after dying, and I noticed that if I left it off for too long, once it woke up it would be in "testing" mode.
So, if turning the TX off will do it, loss of radio signal would probably also do it (from the RX/DMD's perspective, it's the same thing).

D.