I did hair transplantation through FUE, a year ago. Why did my transplanted hairs turn curly?

result is good but with the time as my hairs become longer they get curled and looks unnatural although my hair folicals were taken from my back and sides area of the scalp where the were not curled or curved. what is the reason and cure ? how can i get them straight i m not using any therapy because most of therapy i used in post op had severe side effects .kindly suggest me the best solution.