I am wondering what caused my stuttering.

I am wondering what caused my stuttering. I know there are people who are born with this ailment and there are some who develop it as they grow up, my case is the latter. I remember that I changed my school in 2nd grade and around that time is when the stuttering became so much more prominent. I do not remember when did it start but people account that did not stutter earlier. Over the years, my stuttering deteriorated from 2nd grade to around 5th or 6th and then it started to improve. I had not received any speech therapy, I did most of the stuff through some kind of self psychotherapy and that's what improved me. I do not stutter consistently now, but I do on some points, for example, some sounds like with words starting from S and H. I am sometimes able to speak very fluently, however, there are times when I get extremely nervous and struggle to speak words and have plenty of pauses. Note that there have been occasions when I did not stutter even when I was nervous for example this one time when I had to give a small speech in school assembly while campaigning for school elections but then there is that time when I had interview with the teachers and I stuttered like hell and when I had to present a topic in front of my class and I stuttered so much it broke me down. What is the cause?