It is not clear what you are asking "Why?" about. If you are wondering why the onEnterFrame: start happens first it is probably because the ENTER_FRAME event listener is triggered before the file being loaded has finished being loaded. What are your expectations?
The function can not be interrupted during its execution.
If you remove the nextFrame() method, and put in its place anything except the gotoAndStop() gotoAndPlay() or prevFrame(), then the result is as follows:
I can not understand why the function was suspended. No event can not abort the function before it is completed.