I'm having the same problem, since you posted in 2014, I was wondering if you figured it out yet. It seems to be genetic. The father had it after he got bigger and it seems he passed it on to 2 of his offspring.
I just noticed my Yorkshires (3 months old) are doing the same thing but not my Berkshires. They are my first piggies and I have been researching but could not find anything. I thought they were just stretching. Its like they cannot carry their body weight and their legs just give out.I'm having the same problem, since you posted in 2014, I was wondering if you figured it out yet. It seems to be genetic. The father had it after he got bigger and it seems he passed it on to 2 of his offspring.