The cause of this crash turned out to be as bizarre as it was avoidable. By unfortunate coincidence, the signal link between my RC controller and the Mavic Platinum Pro glitched at the precise moment when I deselected smart (low-battery) RTH while the drone was cruising at 28mph well over 3 miles out and still en route to the turnaround point where it was to fly two concentric circles over a ground structure of interest, before heading back home.
Unsure of whether my cancel RTH command was transmitted due to the ill-timed RC signal glitch, I decided to re-establish RC contact with the drone by scurrying up the ladder of a nearby 60-foot water-tank tower, with my RC controller foolishly left switched on and connected to my iPad that was nestled on the floor of the shoulder-bag slung around my neck, as I did my best primate impression scaling the tall tower. This hurried and somewhat risky expedient had proved effective as a last-ditch gambit several times in the past without anything untoward happening, but things were to wind up going depressingly awry this time around.
As fate would have it, and Lady Luck nowhere in sight, the RC controller did re-establish its signal link with the errant drone at the worst possible moment during my clambering ascent up that water tower ladder, which was regrettably before I reached my usual vantage point and while I still had both hands fully occupied climbing the ladder. The debacle that unfolded next became painfully evident after I finally retrieved the crashed drone and played back the footage documenting its final moments before it flew backward into power lines.
It just so happened that the RC controller which I had hurriedly stuffed in a side pocket of my shoulder bag for that ladder climb, wound up with both left and right joysticks in the fully aft position, meaning the throttle was cut to flight idle, and the Mavic Pro Platinum was in a fully nose-up position, which launched it into backward flight. Those two fully-aft joystick positions were received by the drone for just a few seconds while the RC controller was still crammed inside my shoulder bag as I completed my ladder ascent, but that brief moment was all it took for the drone, whose collision sensors I had thoughtfully de-selected to maximize airspeed, to unceremoniously embark on a rapid decent while flying backward, directly towards the looming high tension power lines as the camera rolled.
By the time I completed my ladder ascent moments later and retrieved by RC controller from the shoulder bag, the crash had already occurred and the drone was lying in deep grass directly beneath the high-tension power lines 3 miles away as the drone flew but a 7-mile drive away, with its strobe luckily flashing and still affixed to its rear-panel attachment location that was scant millimeters below the power line's point of impact when the drone backed into it. Litchi's final position map put me within 50 yards of the drone's crash site after I arrived in my Jeep to hunt for it in an overgrown cornfield, and that flashing Firehouse strobe light enabled a fellow searcher to find the remarkably undamaged Mavic Pro Platinum nestled in tall grass where it evidently made a soft landing, against all rational odds.
The most significant lesson I derived from this unfortunate adventure, is that for anyone who resides as I do in the Third World boondocks beyond the jurisdiction of the FAA and CAA, and who has purchased one of those extra capacity batteries that can allow a Mavic 1 Pro or Mavic Pro Platinum to cruise aloft for 35 minutes as opposed to 23 minutes of a stock M1P battery, it is VITAL that BOTH obstacle avoidance AND low-battery RTH should be disabled BEFORE takeoff, so that intermittent RC signal connectivity does not lead to unexpected outcomes of the sort I described here. Also, it is best to power OFF the RC controller shortly after takeoff and to leave the RC controller off until 5 minutes before its expected time of arrival, which is usually a nominal 5 minutes EARLIER than would be deduced from Litchi's on-screen flight duration estimate.
Provided that winds are calm, programming fully autonomous beyond-RC signal Litchi waypoint missions only requires that the estimated flight time shown on the Litchi Mission Hub flight planning map is LESS THAN 35 minutes when the drone is fitted with an extra range battery. Since autonomous Litchi waypoint flights generally arrive at the home launch point 5 minutes BEFORE the estimated flight duration shown on the Litchi Mission Hub screen elapses, a Litchi waypoint flight estimated to last 35 minutes will complete its programmed sequence of waypoints and arrive at the launch point precisely 30 minutes after the flight was launched, assuming of course that winds aloft are moderate to calm for the duration of that waypoint mission.