Like humans, chins can have seizures for a variety of reasons. The main ones are sugar crash and genetic inheritance. I am seeing more and more seizures in pet homes these days, they go hand in hand with diabetes and feeding sugary treats.
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. This female lived to be almost 10 years old and beyond seizuring lived a normal active life. Her owner did not say what she passed away from.