I had the exact same thing happen on my Sugoi shorts - first time I wore them! It was so bad, it rubbed a hole into my shorts and then started to rub my thigh raw too. Ouch!

In my case, it was due to velcro but I'd had the same bag for years and only had the problem with the more 'fragile' fabric on my Sugoi shorts. I switched out the bag for a non-velcro one that actually sticks out behind me a little further but is less underneath the seat (if that makes any sense). I've also not purchased a second pair of those shorts even though I like the chamois for long rides. Not worth it to me. My cheaper PI's have tougher fabric, so I'll stick to those for now.