What is the cause of chromosomal mutations or chromosome aberrations?

Partly genetics. Some people are just more prone.

Age. Pregnant women over the age of 35 are more likely to have a baby that has random mutations (Down's Syndrome is an example of this).

Radiation. Radiation can cause mutations.

Sometimes there is error in Meiosis, resulting in non-disjunction, meaning that chromatids don't segregate correctly and you can end up with uneven chromosome numbers, as is the case with people with Down Syndrome.

You should also research transposons.

Essentially they're segments of non-coding DNA that can jump around the genome, and this jumping around has some oddly strange effects. For one thing it's been speculated that transposons can play a role in unequal crossing over.

Another interesting thing is that humans and chimps have a higher rate of transposons then most other species. Something to think about in terms of evolution.

Transposons are probably not a direct answer, but it's definitely a possibility.

it can be due to factors such as radiation or heriditary from past generations.sometimes it can be a collection of such traits in the genes whos phenotype is visible in the coming generations  when collected on or when a recessive gene opposes it ,its kinda complicated..........