My kitten did this when he was about 6 months to recently.
For some reason, he always used to dash outside then get scared and RUN from any and EVERY thing (including me!)
I combatted it by keeping a VERY close eye on him when the door would be open, which is kind of difficult due to the fact that I have to keep an eye on my dog as well. And actually taking him out on the leash has helped. He now kind of knows that when the leash is on he can go out, but when it's not he isn't allowed. Also, stomping my feet and the spray bottle has gone a long way. I just try to make sure that the screen is closed or that the door isn't open very much.