It's incredibly bad luck, but I don't think you'll ever get an answer as to why it's happening, or be able to do anything to prevent it. I see it is just one of those things. The main point is now you know what to look out for, and the instant you see that wobbling/weak back leg symptom, presume it's ery, ery-vaccinated or not, and just get a penicillin jab in as fast as you can. Oddly enough, I had a young gilt two weeks ago who was obviously unwell one random breakfast time. and when I finally managed to catch her and take her temperature it was 41 degrees - she had been quite symptomless the afternoon before. She too, like the three last year, was jabbed with penicillin within 2 hours of seeing the tell tale symptoms, and has recovered fully, as did the others. Even by tea time the same day she was zooming around like a completely healthy pig.
From my experience with my 4 weaners, there is no lasting effect, and it is completely curable as long as they are injected with the antibiotic fast enough, although I've read there can be long term consequences if you want to keep the pig for breeding - didn't apply to us. All three of mine who got it before continued to grow normally on to full size at slaughter time, no residual stiff joints or lack of mobility, and there really was nothing to tell by the end of their time that they had had it all.
You were very unlucky losing your first pig who got it, but that was undoubtedly due to inexperience - now fortunately you have the knowledge you need to act fast and keep on top of it. Don't despair too much - all livestock keeping is fraught with unexpected illnesses and problems, many with no explanation at all, however heartbreaking each one is to their owners.