Waited for course lock and home position as usual. Even did a compass cal for the heck of it on Phantom power-up. Was flying at about 1300 ft distance and 150 ft altitude over a lot of houses at night, so I had a good visual fix on the bird. In NAZA mode, both switches up, commanding about 5 mph forward stick. Wifi starts to lose bars. Then I get "Phantom Connection Broken" on the app screen. No biggie right? Wrong. At the same time, I see the bird descending what appeared to be straight down at a pretty descent rate. No commands given to do this. I panicked for a moment, gave up throttle command but still descending. I then switch S1 middle and back up. Control regained!! Bird is hovering and responding to my commands. I could feel my heart beating through my chest. So I switched to course lock and brought it back uneventfully within about 350' of home. Then tested IOC off, CL, HL all fine. Tested GPS, ATTI, and even Failsafe. All fine. I thought maybe the Failsafe mode was Landing instead of RTH Landing, but disproved that with the testing. Flew a second battery without incident. Never thought I would have one of these unexplained behaviors, yet I had one! When I got home, I ran the Assistant. I did an advanced IMU and checked Failsafe Landing was checked. I even checked Landing and rechecked RTH Landing just to be safe. Did a stick cal as well for the heck of it. One more thing, I MAY have had the TX angled up too high, instead of pointing more at the bird. Might explain the connection broken, but not the landing mode. Even if it was trying to RTH, it should have stayed at altitude as I was above 20m. Looked like it may have been going to 20 meters, but I didnt wait to see. Again, I was able to regain control with a switch of S1 to middle then top. Implying it was RTHing and I got control back. Any ideas?