Blaming it on "multiple drops on a hard surface". That's rich. It's a manufacturing defect. I've watched a video recorded by a tech that took one apart to show exactly where the flaw is.
Happens more with the 6 Plus if it gets a slight bend and puts stress on the component which causes the issue. Sure, a small drop can cause it as well, but it's a manufacturing defect that's to blame here.
They should accept responsibility for the defect and fix the affected iPhones for free, same as they do with the faulty rear camera sensor.
EDIT: here's one of the videos I watched that explains it well:
This is a better video that explains the problem: