Why some people having stammering?

It can be hereditary.

Being under stress, overly tired, or having any extreme emotion can make stuttering worse or make it start if you are prone to stutter. Make sure you are getting plenty of sleep; try to destress your life as much as possible; try to avoid situations that cause emotions like anger. The Stuttering Foundation of America is a great source for help. They have information for all ages, online videos, free e-books, and downloadable brochures.