One cell has two copies of chromosomes while the other has none. what could have caused this

The answer should be mutations.

Mutation is where there's a sudden permanent change in the the DNA in cells. it can be caused by faulty nucleus separation or DNA duplication. exposing to chemical mutagens or radiation can increase the risk of mutations.

Either way, one cell that has two copies of chromosomes while the other has none is a result of mutations. this is also to cause of Down's syndrome. if an egg that has a extra 21 chromosome and it is fertilized, the baby born should result in down syndrome.

usually our body has some mechanisms that can stop mutations, but as we get older, the mechanisms can get a little faulty, so the risk of mutation is increased.