Good job tree collision doesn't work
Yeah custom airfields are 'fun' - in that I don't think they were ever properly finished wrt the coding of how AI interact with them.
The reason I think you need a strong wind is the shortness of the runway you've used- the AI are running outside it before sufficiently slowed down to taxi and get 'confused'- unless their groundspeed is reduced by a strong wind as in your mission.
Something I've been messing with
http://www.mediafire.com/download.php?8jzx4zimlg21tfn