My guess is that the knees are blowing out a little, or perhaps the falls you take are putting different strains on the seat fabric than when you聮re on a board. I will say that most snowboard pants seem to be made more with style in mind than function, so it could be you simply need a more serious pair of pants.
Patagonia Primo Pants
Primo PantsA good starting point is Marmot聮s Randonee Pant ($300; marmot.com). These are made with Gore聮s new Pro Shell material, which is lighter, tougher, and more breathable than the previous-generation XCR Gore-Tex. They聮re suspendered pants聴sort of short bibs聴which is a design that works well when skiing. Cuff guards protect the lower leg from ski edges, and overall solid construction should hold up well. Yeah, they聮re a bit pricey, but for top-end pants $300 isn聮t too bad.
You might also like Patagonia聮s Primo Pants ($285; patagonia.com). They聮re not Gore-Tex, but rather are made with a stretch-woven material (sort of a soft shell) with Patagonia聮s very good water-repellent finish. They聮re more highly water-resistant than waterproof, but in cold weather, that聮s enough. And the Primos are VERY ruggedly made, with reinforced knees and seat, bar-tacked stress points, edge guards and more. They also have internal gaiters and removable suspenders.
If you聮re on more of a budget, take a look at The North Face聮s Foundation Bibs ($170; thenorthface.com). These are made from The North Face聮s proprietary Hyvent waterproof/breathable fabric, which is pretty decent stuff. They聮re well made, with reinforcements in all the right places, and should hold up well to telemark skiing.
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