There are various designs of backlights. The all typically have a stack of loose components in an assembly. By loose I mean not totally fixed but not too fixed. They have to free float enought that temperature changes do not cause issues. They also have to not stick, warp, or buckle over time. Harder to engineer then you might think.

So consider what might happen if for example the top backlight film might buckle some then stick to the back of the lcd. The film might deform which would change its optical properties. Then later thermal cycling might cause release. It might do same elsewhere.

Not saying this is mechanism, but just example.