Seizing chin
Like humans, chins can have seizures for a variety of reasons. The main two are drastically over or underfeeding certain nutrients, or more often, genetic inheritance.
This particular chin had genetic seizing problems. The breeder is reputable but had no idea, it isn't something you'd notice unless the animal lived in your living area. All three of her kits started seizing by 6 months of age. They would wobble, collapse or fall off of things, and go stiff like above.