Can someone explain the science behind stuttering? It's such a strange issue.

Can someone explain the science behind stuttering? It's such a strange issue. Why don't some people stutter when they are alone? But in front of other people they suddenly can't speak fluently? Why don't people stutter when they sing or rap or speak in unison with others? Why do some people stutter more around different groups of people? Why does some peoples stuttering get worse with age and some peoples gets better? What part of the brain is responsible for stuttering and is it curable?