There're 3 main reasons that cause the iPhone 6 Frame and LCD Separation:
1. The LCD assembly and frame was assembled by cheap glue, which is not sticky enough to meet the higher requirements as the iPhone 6’s frame is thinner and the screen is bigger than previous models.
2. The iphone 6 frame is thinner than previous models, the machinery as well as the process that is used for previous iPhone models are not applicable for iPhone 6 any longer.
3. Many third-party manufacturers are unable to do the assembly work in a dust-free room. Dust easily sticks to the glass unless these conditions are met, which will affect the quality of the glue. Besides, there’s a varnish on the bottom of the iPhone 6 glass, if it is not sticky enough and coming off, the frame is guaranteed to come off as well.
Check this video for more details: https://youtu.be/2A3bbR_g_vc