Why does a disc become understable?

[the invisible tree]

I used to think that maybe it was the weight coming off the rim due to collisions with trees, the ground, etc. I've got a star TL that I've used quite a bit and that was stable (at my power level) with a bit of left fade when new but after playing it for around three months has become almost roller level understable. It shows practially no wear other than a few streaks on the top of the flight plate. It doesn't look deformed in any way. So, what is the actual physical process at work that causes a disc to become understable?

[Blake_T]

contributing factors:

1) it loses its perfectly round shape.
2) stabiliing characteristics of the rim smooth off.
3) air friction increases.
4) impact shatters plastic molecules weakening the structural rigidity and forces the rim slightly downwards.

[MrTasses]

I think it's the aerobie guy that claims that when the discs hit trees it tends to curve the disc downwards which makes the profile of the bottom edge slightly less prominent. He claims that is why you can push a disc towards overstable or understable depending on how you flex it. (I would think that the flexing would also have effects on the dome in the flightplate that could also contribute.)

I think he needs calipers to measure the changes so just taking a look at the disc won't tell you much about shape unless you're throwing a taco.