In karate we had it pummeled into us that you, "never turn you back to the opponent." Sure, there were rare exceptions. One sensei used to like to feign fleeing from us and then hammer us with a turning back kick as we stepped in - a Parthian shot. But the exception to that rule was very rare.
In aikido and judo we learn that there are times that it is appropriate and effective to turn your back, including hipthrows and under-the-arm techniques - and sure you can get these things to work pretty good with a compliant partner, but then you try it in randori and you get beat up whenever you turn your back.
A lot of this has to do with hesitation. You start to turn your back, knowing that it is the right time and place, then something in your mind says, "better not!" and you hesitate and your momentum and synchronization are spoilt. The folks who can smoothly and in synch just go ahead and walk under that arm or just do the hipthrow more often than not are successful.
One possible solution for this - get more experience at ushirowaza. Get used to having an attacker behind you restricting your motion. Learn how to walk out of this most terrible of mistakes.
[Photo courtesy of Wontolla]____________