They didn't cut in at a sharp angle, they moved slowly, and when his front left hit the crud, it slowed that wheel, which caused increased resistance, like having a flat tire. Because this driver was in two clear tracks, they should have anticipated the pull that was about to occur, They didn't, and the car then veered sharply.
This is a good example of this effect happening, and one many drivers don't know or consider. If you do have to cut across a snow ridge, you want to do it smoothly, but decisively so you aren't on it long. And be ready for the pull. Often when you see a car go into the ditch on the highway it's because they slightly dip a wheel on the shoulder, and just get dragged off the road.
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