It's hard to know. This is one reason I'm very picky about where and from whom I obtain my birds. I also keep records of where the birds came from and where that particular breeder got their birds from....yeah a lot of research and work but the results are I have few problems with my birds and haven't had any diseases in 30 years. Also a strong BIOSECURTY program goes a long way to having healthy birds.
This having been said, I think your probably experiencing a vitamin deficiency. Not just the chicks but it probably extends to the parent birds. I'm a strong believer in if the parent birds are getting a balance and complete nutritional feed, the stronger the chicks will be at hatching, less nutritional defects too! But it's hard to know for sure the birds bodies are utilizing the nutritional value of the feed, until a problem occurs. Some bird's bodies, digestive systems don't develop correctly, thus, when they begin to breed, it will lead to poor survivalbility of their offspring.
I learn this a long time ago. Back in the day it was hard to find high protein feed much less a complete and balanced feed. I had a lot of weak chicks hatching and defects that were not genetic in nature. After switching to higher protein feed those symptoms slowly began to disappear. Back then the highest protein I could get was 16%, I found another source that was 30% and things began to change...then I discovered that that was too much protein, started having digestive problems...it was 'burning them up'. Switched again to 24% protein and the problems went away again.
You really need to look at the analysis of the feed your purchasing, what's on the tag isn't necessarily what's actually in the bag. Most manufactures have a minimum and maximum percentage of protein and vitamins and minerals in their products. Due to operator or mechanical factors those min's and max's may not be accurate.
Sorry for such a long answer but it's not a short, concise question. Falling over, balance problems, walking backwards can be attributed to a Vitamin B deficiency as well as a folic acid deficiency in the hen and to a lesser extent Zinc deficiency.
Hope this helps and doesn't just 'muddy the waters' even more for you.