iphone 6上的iPhone应用程序时不时切换到后台

I am in the process of testing one of my apps. It works fine on iphone 4s and 5. On iphone 6 however, my tester is reporting that every now and then the app goes to background and he need to relaunch. I would assume it is crashing but he is not able to replicate it (which means there is something really wrong).I am in the process of testing one of my apps.