It sounds to me like sudden awareness, you realise that the dream is forming and then become too excited and 'jolt' out.
I advise remaining more passive until the dream has formed more, that way you can get more of a foot in the dream.
Other things you could try would be manipulating the forming dream, so try and change it to your will which may make it less surprising. Most wilders use visulisation to enter the dream, you could try that.

Good luck