The sequence, as far as I can see, is:
The fake lead expands and contracts in sunlight and heat over the years. It gradually warps and raises itself off the glass. Either it starts to sag then or we come along and if we're heavy-handed we loosen it even more. So, it starts naturally but we definitely can make it worse.
Solution: Be gentle with it and, on the horizontal/vertical lead version, run your brush at an angle across it. Having the brush at right angles to the lead seems to maximise strain on it.
Vin