First, thanks to everyone who has commented on this thread. I have leaned much, as I usually do reading TBN. I believe Ford850's test parallels what happened to me. When I had my incident with the tractor, I was on a relatively steep but gentle downhill slope, gentle in that it was short and had a gradual transition to level land at the end. I hate to admit it, but I did not have my seat belt on when the tractor went out of gear. A few days after the incident, I used my machine to finish rotary cutting the rest of my place, some of which has steeper and longer slopes with less friendly or no escapes at the bottoms, so I wore my seat belt. I also keep my right foot over the brake pedal, without riding the brakes, just in case. I had no repeat of the vehicle coming out of gear. In fact, I was very impressed with my TC55's manners on my steep hills, both in stability and maneuvering. I think I will have the wheels moved out to setting C to gain a little more stability and to get more clearance for the TPH. My tires are fluid filled, so I will probably have my dealer to the work as I am not really set up to handle all that weight. I think the seat switch is sensitive to slight decreases in mass, and mowing down a hill with bumps can cause enough of a change to shift out of gear. At least I think Ford850's test and my experience lend credence to this theory. I know one thing: I will always buckle up when using my tractor.
Again, thanks so much for everyone's help and interest. I am sorry to be so slow to post with my latest; we've been busy around here with building a house and reclaiming about 15 acres of overgrown pasture. Hope everyone has a nice Memorial Day.