I worked some with my brother that had his own way of using a string timer. And I don't mean to be rude when I say this. It all come down to how he was using the trimmer. It is about the only way one could turn off the cuter head if it is installed properly.
What my brother did he always (that drove me nuts) was to put the head in deep grass or on the ground then pull back the trigger and give it power after it was in the load. He had poor cutting and took him for ever to make time. He also would do maybe what your doing once he was done cutting and the head was finally spinning at a high RPM he would stick the head in the ground or grass to slow it down. This in essence was giving a revers tork to the head, this would be possible to unscrew the head. The high compression of a new 4 cycle without a clutch when you slow it down with the ground as to stop the spinning your going to unscrew the head.
This is meant to ad not criticize, my brother took little advice from me so I just help him put it back on a few time and excepted it. You think about how and what your doing and if this is close to what I have tried to explain. I hope you have gained some insight and don't have this problem any more.