How do DNA mutations occur?
A mutation is an error during the replication of the DNA when a cell replicates to divide into two. It can also be caused by thing like radiation.
The cause of mutations in our DNA is that sometimes atoms and molecules end up in the wrong place. Such a small event can cause dramatic alterations.
Mutations happen all the time in your body, but most of them are so insignificant that you can't notice. It is only in you are unlucky that the DNA error is noticable.
(Personally I do not know much more myself. I therefore refer to that you google it and find a reliable source.)