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 youre 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.
A good starting point is Marmots Randonee Pant ($300; marmot.com). These are made with Gores new Pro Shell material, which is lighter, tougher, and more breathable than the previous-generation XCR Gore-Tex. Theyre suspendered pantssort of short bibswhich 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, theyre a bit pricey, but for top-end pants $300 isnt too bad.
You might also like Patagonias Primo Pants ($285; patagonia.com). Theyre not Gore-Tex, but rather are made with a stretch-woven material (sort of a soft shell) with Patagonias very good water-repellent finish. Theyre more highly water-resistant than waterproof, but in cold weather, thats 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 youre on more of a budget, take a look at The North Faces Foundation Bibs ($170; thenorthface.com). These are made from The North Faces proprietary Hyvent waterproof/breathable fabric, which is pretty decent stuff. Theyre well made, with reinforcements in all the right places, and should hold up well to telemark skiing.
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