Most times a persons hair will suddenly change from curly to straight due to hormonal changes in your body which is very normal.
So you might start out for years with curly hair and then later on in life it can suddenly change over to straight hair because of hormones changing inside your body.
My brother used to have very curly hair as a toddler and kid and then when he got to 20 years old it suddenly began straightening which he now prefers.
Stress, aging and even muscle tone can also be causes of changes in hair from curly to straight and it's also very normal for hair to suddenly change.