looking at pictures, first thing that came to mind. was bucket was put down, then tractor was put into reverse, and the bucket was lowered more on purpose to cause rear end of tractor to come off the ground.
next thought was, bucket was "to high" and hit a bump, and caused tractor to begin tetter tottoring to point there was more weight on front vs back of tractor and away she went face flat into the dirt.
as far as bucket size, all loaders on tractors i have seen, normally the bucket is a couple inches wider than width of tires. exception a light duty mulch bucket.
might as well add it, because first thing i thought, just like so many others is rear end weight. FEL (front end loaders) sticks a good amount of weight out in front of the tractor. something heavy from a rotatory cutter, to heavy duty box blade, perhaps even a rear scrapper blade, to a back hoe, to 3pt hitch
ballast/weight box.
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as far as the FEL itself. some others hit on it. but you really do need to try and keep that FEL down low to the ground. when you move it all the way into the up position. you can quickly tip over to one side, or like what you have already done.
only time i have ever needed to raise the FEL bucket high, was to help move some stuff from ground, to top of roof. or i had to raise bucket so i could turn around, and not hit the fence posts. but as soon as i was done the FEL went right back to the ground, and hovers a couple inches above the ground. there is another exception, when i had some chains on a couple things. to lift them and put into back of a pickup truck. but all of these doings have been on flat even ground. and going a snail speed of movement. to reduce chance of hitting a bump or wheel going down into a rut and causing me and tractor to tip over.
also try to go up and down hills. and try not to go across a hill horizontally. to easy to hit a small rut or hole (hole from a varment, to a dog, to your own previous tire tracts) which could send tractor over on its side.
as others have said, do not go 100 MPH with pedal to the metal attitude. take it easy, take it slow. and over time build up speed as you get practice / experience.