Well, I can't really explain it, and knowing next to nothing about meat chemistry, the results are definitely mystifying, but the result speaks for itself.
I have been using a fairly high-end pork, which I get from a restaurant supplier. That pork was used in all of the bratwursts where the casings shredded.
I threw all of that pork away, went to a Ranch 99 market (a California Chinese supermarket chain) and bought fresh pork shoulder over the counter. Took it home and made a new batch of bratwurst. Same recipe, same casings, same technique. Shazam! Juicy, delicious bratwurst, casings perfectly taut and intact.
Can't explain it. I have used the original pork (from the restaurant supplier) before with no problems. I must have gotten a bad batch or something. It looked good and smelled good, but something was clearly wrong.
As to how "bad" pork could make casings disintegrate, I haven't got a clue, but switching the pork DID solve the problem.
Thanks everyone.