It's all the abnormal stuff... like falling. Or if you are in 3D snow, you may get your legs pushed together at different levels when you screw up.
Another factor is how wide your skis are. The wider your skis, the more likely you are to hit your pant legs with your edges.
I've slice through non-reinforced pants within 10 days. That said, I do a lot of off-piste skiing with my 110-mm skis, often in low visibility.
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