The actual cause is suspected to be the way Apple changed the the way the touch control chips are mounted to the board and also Apple removed a metal shielding plate too. With the way phones tend to subtly bend and and a combination of heat the exposed chips end up coming loose. Theres a decent video here explaining it and how people fix it.
Actually its a design defect to do with iPhone’s 6 series and the chips controlling the touch controller of the screen. Apparently iPhones affected by it end up with a flashing grey bar and being unable to use the touch screen. Apple is currently being sued for it.