You figured it out. The tractor was transversing a swale and the angle got to be too much and the tires lost traction. I have a tilt meter in the cab of my BX23S and when the tilt meter gets to 5% I stop and reposition the tractor. At 10% I get really scared of going over, and at 15%, well I have never made it to that degree. The common denominator in all of these accidents is operator error.
I purchased my meter at
www.tiltmeter.com and it is well worth the small cost.
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I think you will be pretty safe using your tilt meter that way.
Back in olden times when tilt meters first became popular for rock crawlers, we saw people tilt their rigs under controlled conditions (with flex ramps in a parking lot) and then use that tilt number as a hard & fast rule as the number they could safely tilt offroad.
Then when they got in the field, they promptly rolled over, usually at a higher rate than they did before they got tilt meters.
It became apparent that very minor differences in loading, tire pressure, fuel level, alcohol intake, and phases of the moon all had an effect on stability that often rendered that hard & fast number useless.
Most of us got rid of tilt meters, and went back to relying on the seat of the pants tilt meter.